I'm going to start off this review by stating that this review is focused on the areas of the book I've used, or have extensively read over. There are some sections I glossed over, simply because they would never come up/effect any game I'll play. This isn't to say that these sections aren't of value, simply that they did not fit in the campaign I'm playing in.
So let us get to it, shall we?
I came to the Grimoire of Lost Souls for two things. That was the Pact Maker class and of course the spirits that they use. The pact maker class at first glance could look a little bland, however, it ends up being a solid chassis that allows for a fair amount of versatility and variance for different builds. Pact Augmentations, in particular, stand out here, as they give you a decent amount of choices to either patch up some weaknesses, or reinforce strengths when you receive them.
The secrets available to the class are somewhat hit or miss in my opinion, with only a handful that I really wanted to use. However, with the implementation of one particular secret that allows you to choose an occult feat instead, this was less of an issue. This is personal tastes, however, and I'm certain that there are those that would find many of the secrets desirable.
One thing in particular that I didn't care for, was that pact makers (and all other archetypes and prestige classes) do not automatically gain any spirits when leveling. Since the process to learn a new spirit is designed to take a few days, that means that while everyone else in your party is automatically getting new stuff, you still will have to wait a little bit, hoping that in game you're allowed the time to make the appropriate checks to get that cool new spirit you've had your eye on. This was meant to push the flavor of the class, with the intention that the GM might flesh out a miniature story of discovering this new spirit in the game. That said, my GM and I found this idea to be a bit tedious to implement into a long campaign, not wanting to add a veritable side quest every time I wanted to gain a new spirit. This, in turn, boiled the flavor away down to the mechanics, which is just rolling and trying to hit certain DCs. My GM was gracious enough to homebrew the allowance of 1 automatically gained spirit that my character is capable of using when I level. This has worked out for me so far, and with the other rules in place, has left the balance of the class undisturbed. Add to that, that I can still work to learn more spirits over time, and I'm still a happy camper.
Now onto the meat of the book. The spirits. There is a lot of flavor and heart poured into these spirits. A lot of interesting and/or fun effects. The fact that a pact maker can mix these abilities together as they progress, makes them almost a Swiss Army Knife able to pull off all sorts of different feats, potentially. From reincarnating into a fish person if you die to survive while your head is cut off, to transforming into animals to mimicking spells, and on and on. Some spirits even take major influence from characters and stories we know and love today, which just makes them that much more fun and inspirational.
There are a good chunk of spirits that are situationally useful, but there are just as much, if not more that are generally useful and versatile. I found it extremely important when playing this class to know which is which and to use the generally useful spirits unless you find yourself in the situation in which the situationally useful spirit you've learned arrives. To this point, I even went as far as to making a guide for myself to realize the difference and have found this mindset to be helpful in making sure I can feel useful and do things in any given session.
The feats provided in the book are more or less solid. Some can be situational or are meant for very specific builds, but that is to be expected and I don't blame them for it.
As far as magic items go, there weren't many that excited me, but there were a few that I liked. Those tended to be either too expensive for my character to use at the time, or simply help me manage to make good pacts with spirits. Other magic items seemed to be meant for campaigns that were steeped in this books flavor and setting, and so won't be seen in my campaign, as I'm the only one playing a pact maker, using these spirits.
I won't review prestige classes, as there weren't any that I felt interested in using, and so I only glossed over them. I also haven't used any of the spells added to the book, nor the special aging rules added to the book so far, and thus cannot give a reliable review here either. Lastly, since I chose to play a straight up pact maker, the archetypes and archetypes for other classes (such as sorcerers, fighters, rogues and such) never came up for me, and again I do not feel I can give a reliable review in this regard.
The art of the book is solid. I enjoyed the seals drawn above each spirits entry the most. To be honest, I came to this book more for the function than the flair, so I've not really bothered to look over the artwork in detail.
In the end, the pact maker character I made using this book, along with the spirits provided has been one of the most fun characters I've played in quite some time. Now, a big chunk of that enjoyment is the personality I've made for them, the setting and the campaign that my GM has developed, but the versatility of the class, and the flavor and various abilities the spirits offer are a big part as well and I can't think of wanting to play another class for this particular PC I've developed.
TL'DR: The class, spirits, and feats provided in this large book are largely very solid, and very fun to play, and very balanced, with only one or two personal qualms that were easily be mended with a very simple house ruling. Very good stuff. Would definitely recommend to others.
