Must Have Player's Handbook 2

The Players handbook II is the best source book I have ever bought. Period. It offers a huge amount of content in a concise package, none of that filler prestige class padding.
There is an astonishing 8 base classes, updating such classic classes as barbarian, bard, druid, and sorcerer. All new classes introduced include: Avenger; a divine striker, Invoker; a divine controller, Shaman; a primal leader, and Warden; a primal Defender. Also a new power source, which is featured prominently, Primal is added. Essentially you draw your power from nature and the wild, like a druid.

There are 5 new races, bringing back some old favorites, but also adding some great original IP. Half-orcs are revived, they get +2 str and +2dex. Devas are a new race described as "Similar to humans, but with an unearthly beauty and uncanny stillness about them." They get +2 intelligence, +2 wisdom. Gnomes are revised with +2 intelligence, +2 charisma. Goliath are a new race which are essentially very tall, Grey, dwarves. They get +2 str, +2 con. Shifters have two possible builds of either +2 str, +2 wis, or +2 dex and +2 wis. Also new paragon classes called Racial Paragon Paths are added which give you new powers that build and expand upon your basic racial powers.

The feats chapter is great, again, bringing back some old ones and adding great original feats.

New magic items expand on the poor selection in the player's handbook
There are new Epic destinies, but I haven't had a chance to look at them.

The players handbook 2 features most of the omissions of the original phb. I was reluctant to convert to 4.0 mostly because of the omissions, especially barbarian, but when I heard that the PHB 2 would include it I converted without hesitation. It is slightly annoying that Wizards is making me pay for base classes which should have been in the original PHB, but this book is so good that I pay gladly. It fills out the fairly skimpy class and feet choices with exceptional new material, all of which can be useful to player.