This pdf is 22 pages long, 1 page front cover, 1 page SRD, 1 page editorial, 1 page advertisement, leaving 18 pages for the monsters, so let's check them out!

First of all, I do know that this book has been published in the very beginning of PFRPG and thus suffers from some problems of the first publications for any system. I picked this along the other books up at a fire-sale and thus didn't pay full price. The 3 fell beasts-files have since been lying around on my HD and I only recently unearthed them. So, how well has this book aged? Let's take a look at the critters:
Air jellies are swarming critters that gather in swarms. I didn't get they are not poisonous and they lack the swarm traits they ought to have. The blistering demon has a cool aura, but the mechanics could have been clearer - as written, once you catch fire you can't extinguish it for 1d4 rounds. While sound in theory, I don't see it working in game due to means to quench fires. Additionally, the abilities are too fire-centric and non-damaging for a CR 15-enemy. Undead-creating Canopic Jars are the one monster in this book I really liked - great idea and they haven't been done yet. Dread Waif is yet ANOTHER take on the creepy child. Although I admit to liking the fluff, I was not impressed by the mechanics: Fear and Despair? Yawn. But wait: The radius of the despair-aura is 1 MILE. This fey (not undead, mind you) can literally drive whole settlements insane in a matter of days. While I don't like that it's a fey (just feels undead to me), the creature DOES make for a cool plot-villain due to this fact. Ghissics are wasp-like creatures that implant deadly larval eggs. I've seen that trope done better and creepier in e.g. 3.5's Hyperconscious. Greenmold Bones are undead infused by druidic magic. When hit by weapons, they penalize the weapon. Why this moldy infestation cannot be treated by anything but a craft-check, I don't know and frankly don't get. The ability feels weirdly disconnected from what should probably be a kind of poison or disease aura. Mercy Flowers trap their victims in illusory worlds and make for nice fey plants. Riverwraths are rather bland aquatic, troll-like fey. Shisa are a take on the foo dog/lion and lack a signature ability - yes, they are ok, but I felt they missed something unique. Finally, there's the Wicker Man. "AAAAHHH, NOT THE BEEESSS". Sorry. Couldn't resist. The problem here is, that e.g. in contrast to the 3.5-Fiend Folio version of the creature, the creature can't imprison enemies and gains no benefits/aura for being set ablaze are given, which I always considered the coolest thing about the creatures. Also, it's a construct sans creation notes.

Conclusion:
Layout is rather subpar, with several pages only half-full with text, resulting in wasted paper, especially annoying due to the b/w-parchment-look in the book. Most creatures could have used another pass at editing - the job was done shoddily at beast, with some typos, punctuation errors and more importantly, problems with the stat-blocks and abilities, rendering the use of the creatures harder than it ought to be. The b/w-artworks are mostly nice, though there are some rather boring ones among them. *cough* Mercy Flower, Riverwrath *cough*While the ideas for the critters almost all rock, I found a problem with either an ability that didn't strike my fancy mechanics-wise or a formatting glitch. More importantly, for the price you can get several vastly superior similar products - Book of Beasts: Monsters of the River Kingdoms just as one example. If you're willing to pay a bit more, I'd suggest Urban Creatures & Lairs by 0onegames or one of the excellent books by Alluria Publishing. Thus, in spite of the good ideas, my final verdict will be 1.5 stars, rounded down to 1 for the purpose of this plattform.

None of these monsters seem inspired, or like something I would have to use, and they don't particularly follow a theme, but at the same time, they aren't bad monsters, and it never really hurts to throw some monsters at your PCs that they have no idea about.

Ok, hitting the creatures one by one:

Air Jelly: a small formatting error, the swarm traits are missing from the creature description (compare to the bat swarm in the bestiary). Other than that, mechanically sound.
Use: I’ll use these in the darklands, definitely adds to the ‘alien’ nature of the underdark.

Burning Demon: Icky monster. The table entry lacks the range of the darkvision. Also it can be inferred, but is not stated, that the grease aura cannot be focused on one target, it just affects the floor. As mentioned above, part of the stat-block template was tossed in as well. Burn is also unclear about adding to the duration.
Use: Interesting monster, I’d save them for a Yomi themed plane, and remove the demon subtype. Or (using the info in the Bestiary) they may be formed from the souls of gluttons.

Canopic Jar: Mechanics look sound, thought Extra Turning should be Extra Channel. It is corporeal, but lacks a strength score. Also an accidental use of the enter key is noted above.
Use: Easily the best monster in here. While the idea seems simple (It’s a brain, in a jar!) the concept of having a monster subtly buffing the zombie-as-a-mummy schtick makes this interesting.

Dread Waif: Stat block looks good. I do wonder if the Fearsome Aura is a (SU) or (EX) effect, and if it’s mind affecting. It’s nice to see the gnome tie.
Use: Will see use in an urban campaign, especially with a gnome PC or NPC.

Ghissic: Stat block again looks good, though I don’t know why Weapon Finesse has a qualifier of (proboscis). Creepy bug creatures.
Use: works best in the isolated creepy town trope, but will likely see use in my games.

Greenmold Bones: The concept and mechanics are sound. I don’t like the fluff for Mouldering form (why does it affect steel) but the mechanic is nice.
Use: Good for a surprising druidic undead

Red Mercy Flower: Truly a monster For the Man Who Has Everything
Use: Best saved for use against aliens and totem based characters.

Riverwraith: Another ‘twisted fae’ As I liked the sundered women from the Scarred Lands, these appeal to me. A duration on the Waterspout is missing though.

Sisha: A good creature, nice to see the foo dog/lion again. The use of ‘usually’ in the description of ‘frightful aura’ is disconcerting. When isn’t it ‘usual’?

Wicker Man: Nice solid construct, though creation notes are missing.

Overview: There are 6 monsters that I’d rate highly enough to use, two druid related creatures, and one that I dislike.

I'll recommend it for new ideas and surprises for the players.

Let me start by saying that if they fix the editing of Fell Beasts Vol. I then I'll happily give it 4 stars. The monster ideas are great, the art is good, and there is a nice mixture of CR's and types. It could have been so good.

However, 1st monster Air Jelly: "feed off of ... plants" - ok, one slip of grammar and an extra space I can forgive.

Blistering Demon - ooh, sounds great! "Reach 10 ft. (Don't list entry if" What? They left part of the TEMPLATE in the stat block and didn't spot it! Also on this one, inconsistent use of 'Burn' (a SA) and 2d6 fire, missing space in 'a25%' and no hint as to whether 'Burn' stacks with itself. This is just sloppy.

Canopic Jar - a line break in the middle of 'hold person' that makes the stat block hard to read.

Ghissic - missing blank line between 'Hive Mind' and 'Host'

Mercy Flower - 'Cha 15' split over 2 lines.

Riverwrath - A missed opportunity to provide a way for a good-aligned party to redeem this poor fey.

For me, Adamant has now established a reputation of throwing unfinished products (this and Tome of Secrets) out of the door to be 'first' at the expense of proof-reading and editing. It makes me wonder if these monsters have been play-tested. I'm also regretting pre-ordering 'Warpath' - I was lucky that the first printing of Tome sold out at Gencon and the pre-orderers got the fixed hardcopy.

I really want to like these monsters, and I'm being so scathing here mostly in the hope that Adamant will read this and improve their QA - there are people here on the boards that would review and edit for them for FREE after all!

Come on guys, fix this PDF so I can post the glowing review the monsters deserve.

As an aside - it may not be in Paizo's style guide, but a subheading between the end of the abilities and the start of the monster 'fluff' would make monster descriptions a lot easier to read.

I haven't had a chance to run most of these, but I threw a Canopic Jar in our session today, and let me say that it was well worth it - they're very cool, and quite possibly my favorite creatures in the volume.

The other monsters all look pretty interesting as well, and I'm looking forward to using most of them in future combats. The Riverwrath is gonna be my next target, since she looks fun with her waterspouts and whatnot.

All in all, a solid PDF for anyone looking for a few new creatures to keep the PCs on their toes! Even if you didn't like Tome of Secrets, don't let it sully Adamant's reputation on this one; there's a few editing errors, but nothing exponentially large enough to prevent you from understanding anything or playing it. Besides, its only 5 bucks!