The first book I would buy is:
The C++ Standard Library - A Tutorial and Reference
By Nicolai M. Josuttis
Published by Addison-Wesley
ISBN: 0-201-37926-0
Cover price: $US 59.99

This is indeed an excellent introduction (around 800 pages), and covers the various container types (lists, maps, vectors etc.) plus the algorithms (sort, etc.), function objects, strings and a whole lot of stuff.
There is also:
STL Tutorial and Reference Guide - C++ Programming with the Standard Template Library
By David R. Musser and Atul Saini
Published by Addison-Wesley
ISBN: 0-201-63398-1
Cover price: $US 44.95

This one is 400 pages. However if you have the Josuttis book I think it is not really necessary.
I also have:
Effective STL - 50 Specific Ways to Improve Your Use of the Standard Template Library
By Scott Meyers
Published by Addison-Wesley
ISBN: 0-201-74962-9
Cover Price: $US 39.99

This has some interesting tips and covers some more obscure points (260 pages).
However I really recommend the Josuttis book first and foremost, and look at the other ones once you become more familiar with STL.