The feel of this book is quite dark, and will specially be suited for dark-themed worlds. A game of Call of Cthulhu could certainly benefit from it, although the spells would need a bit of changing.
Taking each chapter one by one:
Chapter 1: Stress
An interesting idea, but a bit wierd in its mechanics. It brings in a new secondary ability score, representing a character's resistance to stress, as well as rules to determine how great a stress an encounter induces, and what the effects of great stress can have on a character. Could have been better.
Chapter 2: Personality
This was by far my favorite, and I'd have bought the book for this one alone. It presents a way to create deranged character mentalities in a rather quick way (once you understand the rules of course). There are a variety of personnality aspects, each possibly being more or less severe in someone's personality, and each includes a series of traits that describe a specific quirk of the character.
There is also a section detailing a method to understand interactions between different characters, depending on their mentalities.
This section contains barely any d20 rule, because it is not really needed. This means you can easily use this section for any RPG!
One thing that nearly frightened me is that I found some traits that really fitted me very well. It was a kind of self-psychanalisys :)
It looks very coherent and scientific. I grant you that I'm no specialist in this field, but I seriously wonder if the author is a doctor in psychology!
Chapter 3: Insanity
This chapters details various insanities, along with their game effects, such as anxiety, pschotic or sleeping disorders.
The details are very interesting, but the game mechanics are really weird. They don't look very uniform, and I'm quite sure the DM will need to adjust some things.
Chapter 4: Spells
This section contains a spell list and then their description. There are nearly 50 of them and they are all more or less mental-related. There are spells for all levels, though unequally distributed.
First noteworthy aspect: most spells are from the enchantment school, quite a few are illusions, with just one from divination, one from transmutation and one from conjuration.
A lot modify the target's behaviour, or make it see things that aren't. Some also have physical effects, as a result of a mental disturbance.
A few spell names that are quite evocative: phobia, insomnia, creeping shadows, mind probe, mass hysteria, masquerade, etc.
Second astonishing aspect: all spells are for Wiz&Sor! Nothing for the others! Not even the bard?!
Obviously, some spells could be appropriate for other spellcasters, but again, the DM will need to assign those, thus some more work is needed.
The spells seem interesting though.
Chapter 5: Prestige classes
The dreamweaver, the lunatic and the mentalist are the three presented classes.
The dreamweaver gets to do all sorts with dreams and the dream world...
The lunatic draws power and abilities from his state (could have been a template in some regards, especially since this is more a condition than a training).
The mentalist is some kind of inquisitor getting to manipulate other's mentalities.
These are medium quality, nothing groundbreaking, balance looks okay on paper. There are quite a few abilities a character can get from spells anyways, so some of their levels look superflous to me.
Overall, the book is well organized and the text is on two colums.
The artwork is mediocre. There are a few spelling errors. But nothing really distracting.
One of the two main gripes I have, is the way game mechanics are treated. I sometimes wondered if it wasn't some 2nd edition stuff. Some rules just dont look coherent in regards to the other, or just plain weird. On the other hand, these could be left out in a lot of cases, since a lot of the material is here to give the DM additional tools to role (not roll) play characters, set the mood, and find inspiration for really disturbing encounters.
The other negative point is the frequent reference the document makes to another product published by the same company, the Primal Codex. I don't own that one, so it kind of frustrated me. But at the price these are, I can't really complain.
As a conclusion, I will say that I really liked this book, and hope to use a lot of chapter 2 and 3, as well as some of the spells. For $5, this is really good value, considering the good quality of the non-mechanic aspects of the rules.
I give it a little 4 out of 5 stars, mainly thanks to chapter 2.