Yeah, checking out the SRDs should give you a good idea of which non-core books you'd like to obtain. As MarkAHart said, there are also the pdfs, which are quite reasonably priced.
For my part, I've found much to like in all of the major non-core releases thus far: the Advanced Player's Guide is a must (hooray, archetypes!), and both Ultimate Magic and Ultimate Combat offer some integral options, albeit more specialized. UM, for instance, provides some great archetypes for the non-core base classes, and UC is a must if you're into monks.