A correspondent asked for book recommendations — where do I get ideas? Actually, I get most of my ideas at conferences, in The Mathematics Teacher, and sometimes on the Web — not so much from books any more. And of course, most importantly, I get ideas when trying to think differently about something I teach.
Still, I did compile a list of favorite / most useful books and software some time ago. It reflects my own interests and priorities, but you may find it helpful. Check it out!