The Player's Companion is the first sourcebook for the Adventure Conqueror King System, or ACKS. While technically a complete RPG, ACKS was a fairly slim book, so there weren't a lot of options for characters beyond the usual. The Player's Companion goes a long while to addressing those lack of options offering a huge amount of new character classes, new spells, and rules for creating both new classes and spells.
While ACKS is largely inspired by Basic and Expert D&D (called B/X), the player's companion introduces several classes taken from the advanced game: the paladin, barbarian, ranger, and even the anti-paladin. There's also the venturer (essentially the merchant from B/X D&D), the shaman, the witch, even a warlock (not like the 3.x one). Gnomes get one class, the trickster, elves and dwarves three more each. And there are three classes unique to the world that ACKS is set in, two for humans of a certain culture, and one for a lizardman like creature.
One of the drawbacks of games that use "Race as Class" is that you need a new racial class for every archetype (that is, fighter, thief, cleric, etc). You want to play an elf, fine. Want to play an elf cleric? You'll need a class for that. This is further compounded by a lack of multi-classing, so you need a new class for every combination of race and archetype.
While there are 50 pages of classes, it's impossible to cover everything, so the rules on creating custom classes is very useful. For the most part they tread the usual ground in old school class design - the more abilities a class has, the more XP needed to gain levels.
This certainly works, but due to the low level cap in ACKS (of 14), at higher levels, more powerful classes will have an advantage, simply because they can keep growing in power, while a less powerful class will simply stop at 14 and never get any better. And since many ACKS games start at high level, this could be a problem.
The book is rounded out (padded?) with a few pages of mostly pointless stuff, like rules on aging, some more equipment, and a price table for building traps in a character's stronghold.
The layout is the same as ACKS, clean and easy to read. The interior artwork is much improved
I found ACKS to be somewhat "meh" as a game, essentially just a rehash of B/X D&D with a few changes (some good, some bad) with a slight dash of Companion D&D. The Player's Companion goes a long way to making it more distinct from other B/X retro-clones. If you like ACKS, it's certainly a must buy, not that it is essential for play, but because it adds a lot more interesting options to players and gives you tools for building more options.
If you aren't an ACKS fan, but do like B/X D&D or Labyrinth Lord, many of the classes can easily be ported to it.