Released as a PDF-only product, this work puts all the information a PLAYER needs into his hands, whilst excising all of the GM-related material in the core rulebook. It has an advantage in being cheaper than the full rulebook, and keeps at least some GM secrets away from prying eyes (assuming, that is, that only one person in your group ever wants to GM!). The text is basically the same as the player parts of the core rulebook, so if you have that already, there are no juicy 'player secrets' here!
The Introduction explains the rationale behind the game, basically a desire to take all the things that made that classic horror game Call of Cthulhu so great and fold in Pelgrane Press's GUMSHOE ruleset that is specifically designed for any game in which the investigation of clues forms a major part. The underlying idea is that if a clue is important to solving whatever mystery has been put in front of the players, they WILL find it no matter how badly they roll the dice (and often, even regardless of how off-beam their investigation is...). The real joy of investigation comes in going over the clues and making sense of them after all, not worrying if you've actually found them all yet!
The introduction also talks about the two possible styles of game - 'purist' or 'pulp'. The 'purist' style is a philosophical approach to horror where the very discovery of 'whatever' endangers both the investigators and innocent bystanders and there isn't really anything that can be done about it, and going mad is not a matter of if but when. The 'pulp' style takes a more robust approach with desperate actions that might, just might, avert whatever horror is the threat (but might equally well result in death or madness for the party). It's up to the group as a whole which emphasis they prefer, most like a mixture of the two. Something to discuss with the Keeper (GM) before the game begins.
Most of the rest of the book is taken up with how to create your character or Investigator. Anyone can become an Investigator, usually by becoming interested in those Things That Should Not Be (although some are just caught up in it), so you begin by deciding on what job or Occupation you had before you started investigating... some characters may still work at it between investigations, others may dedicate themselves full-time to this pursuit. Whether you still do it or not, it confers a host of knowledge, skills and other abilities on the character. There are also ideas about how someone with that particular professional background will approach an investigation of strange events, the usual route in to discovering the latest Mythos plot. All these Occupations are set in the classic 1930s era, and there are interesting sidebars about how faithfully you want to model ideas of the times... in the real 1930s, for example, female doctors were rare, indeed ladies were expected to do no more than run a home and raise children. Many gamers will not want to constrain themselves to a slavish following of historical reality - and when you are accepting the existence of mind-bending tentacled monsters, you may well feel happy to kick over the bounds of the attitudes of the time as well. Others don't (I recall a Call of Cthulhu Keeper once questioning a doctor turned novellist character I put before her with "A woman would not have read medicine at Harvard"... she hadn't realised that although I, the player, was female, this time I had created a male character!), and it's up to the group to decide what will work for them. Many great 'pulp' characters defied convention.
We then look at motivations or Drives, used to define the character (and also help him stay sane in the face of adversity). They also help with role-playing, shaping the character's attitude and approach to life. Throughout, symbols indicate which choices are appropriate for 'pulp' or 'purist' games, depending on how far to one end or the other of the scale you intend your game to be. There are lots of notes to help you too. Then it's on with detailed listings of all the abilities (that is, skills) and how they work in play, indeed how they can be used to good effect. This section ends with a sample character sheet, and there have been plenty of examples of character generation as illustrations along the way.
The next section, Clues, Tests and Contests, explains how you use all those numbers on your character sheet to good effect. There's loads of detail here and well worth the reading. It includes combat, of course, but brawling is of lesser importance in this game than in many. For a start, it's pretty deadly whic a realistic approach to firearms and other modes of dealing out injury. And most of the time, the average Mythos monster is less bothered by a hail of bullets than you are by a swarm of midges. Yes, there will be opportunities to fight (cultists, for example) but your combat abilities are not going to save the day. Still there is plenty and enough about combat to keep anyone happy (and of course it is of more import the more 'pulp' your game becomes. Of course, this is a Mythos game so a substation section is dedicated to sanity and madness, with many interesting ways for your characters to lose their marbles. Some may even recover...
There's a section on Technology, Weapons and Equipment, based squarely in the 1930s and finally a section of good advice, Putting it All Together. This provides hints and tips for role-playing and for how to conduct an effective investigation. Right at the end is a blank character sheet and some quite disturbing pictures of the authors.
This contains everything a player might need to play Trail of Cthulhu and could be regarded as well-nigh essential for every member of the group.