8 pages total, 1 for the cover and most of the introduction, and 1 for the credits & OGL. Nice evocative cover, and very good to great artwork inside.
There are two new base forms, the hut and the ooze (both pretty self-explanatory).
New 1-Point Evolutions: Air Mastery, Centipede, Earth Mastery, Guise, Hidden Compartment, Hyboreal, Ink Cloud, Luminescence, Metal Skin, Mimicry, Quills, Scintillate, Shell, Sprint, and Water Mastery.
New 2-Point Evolutions: Bloody Spray, Camouflage, Corrode Metal, Death Roll, Deafening Roar, Drench, Formless, Hiss, Howdah, Metamorph, Mucus Cloud, Puff Up, Stench Musk, and Suction.
New 3-Point Evolutions: Adhesive Lash, Darts, Dolorous Wounds, Offensive Digestion, Self-Destruct, and Suffocating Slime.
New 4-Point Evolutions: Gaze Attack, Lava Puddle, and Spores.
The evolutions seem both reasonably priced and well-balanced, and offer a lot of new ideas for eidelon flavor/RPing. Combined with the existing Advanced Players' Guide and Ultimate Magic, it should give a summoner player plenty to work from. As long as GMs and summoner/eidelon players alike remain vigilant in making sure eidelons are constructed according to the rules, there shouldn't be anything in this supplement that will overpower the game. Perhaps the biggest risk is it may induce the other players to want to play an all eidelon adventuring party.
I'd definitely like to see a future Volume 2, with more evolutions, new eidelon models, and maybe a variant summoner archetype or two.
Haven't been able to use it with a gaming group yet, but it seems very solid and jives pretty well with what I'd already deduced from the race creation and breakdown sections in the (WotC) 3.x DM Guide and Savage Species books. Not sure if this Pathfinder version still uses the 3.5 SRD monsters for the calculations or the newer PFRPG Bestiary... it might slightly change some of the values. It could also probably benefit from a couple examples in sample new races building or a sample breakdown of a few of the standard PC races.