The Expert's Handbook is probably the weakest of the printed supplements for True20. It's got a lot of useful options and ideas but the implementation of those ideas is clunky and a bit at odds with the baseline of the system.
You can see a number of reviews (and some responses by the author) in
this True20 thread on Green Ronin's True20 forum.
I'd actually recommend the bestiary, if you're a Narrator (GM). Even if you don't use the creature entries, the book is a useful resource for traits, concepts, conversion guidelines, and examples of powerful creatures.
Just stay away from the equipment list in the appendix.
The Adept's Handbook is generally a better book, though obviously it has a different focus.
Worlds of Adventure is very good, as far as options go. The settings in it are quite interesting, have a host of options, and can all be easily stolen from to enhance an existing or upcoming campaign.
Another option is a Pocket Player's Guide (or two) if you expect to need to share the rules with players a lot. It really helps having multiple copies of the book around.
Good Luck!