The core book, Clock & Steam and Temple of the Frog are my favorites.
Dungeons of Castle Blackmoor is probably the one I like the least, but then I never cared much for mega-dungeons. It does have some interesting features though.
If you are getting into Age of the Wolf, all of the 3.5 books may be useful as background information...
Havard
For geek completeness, you need the core book and the Castle Blackmoor book. I like the core book more, but I've never really used either.
There's a 3.5e Temple of the Frog? Cool, I need that too.![]()
On WotC's site there is Adventures: Return to the Temple of the Frog
I think it was meant for d20 modern... but I think it's actually written for 3.5.
There is a d20 modern page for it here Adventures: Return to the Temple of the Frog
I think they are the same.
On WotC's site there is Adventures: Return to the Temple of the Frog