How to find rocks worth tumbling?
Any stone quartz rich and smooth not porous (no visible holes) like agates, quartzite or cherts are you best bet for getting a nice tumbled stone.

A great book is:

Gems and Minerals of America: A Guide to Rock Collecting (1st Edition)
by Jay Ellis Ransom
Hardcover, 705 Pages, Published 1975

ISBN-10: 0-06-013512-3        / 0060135123
ISBN-13: 978-0-06-013512-6 / 9780060135126

Your local library might have a copy of it.
See to find on sale on Internet also: used ~$5.00

http://www.gettextbooks.com/isbn/9780060135126

BTW, tumbling stones and rock collecting led me to a career as a geologist.