Best introduction to C++ template metaprogramming?

Andrei Alexandrescu's Modern C++ Design book covers a lot of this and other tricks for speedy and efficient modern C++ code and is the basis for the Loki library.

Also worth mentioning is the Boost libraries, which heavily use these techniques and are usually of very high quality to learn from (although some are quite dense).

Modern C++ Design, a brilliant book and design pattern framework by Alexandrescu. Word of warning, after reading this book I stopped doing C++ and thought "What the heck, I can just pick a better language and get it for free".

[Answering my own question]

The best introductions I've found so far are chapter 10, "Static Metaprogramming in C++" from Generative Programming, Methods, Tools, and Applications by Krzysztof Czarnecki and Ulrich W. Eisenecker, ISBN-13: 9780201309775; and chapter 17, "Metaprograms" of C++ Templates: The Complete Guide by David Vandevoorder and Nicolai M. Josuttis, ISBN-13: 9780201734843.

Todd Veldhuizen has an excellent tutorial here.

A good resource for C++ programming in general is Modern C++ Design by Andrei Alexandrescu, ISBN-13: 9780201704310. This book mixes a bit of metaprogramming with other template techniques. For metaprogramming in particular, see sections 2.1 "Compile-Time Assertions", 2.4 "Mapping Integral Constants to Types", 2.6 "Type Selection", 2.7 "Detecting Convertibility and Inheritance at Compile Time", 2.9 "NullType and EmptyType" and 2.10 "Type Traits".

The best intermediate/advanced resource I've found is C++ Template Metaprogramming by David Abrahams and Aleksey Gurtovoy, ISBN-13: 9780321227256

If you'd prefer just one book, get C++ Templates: The Complete Guide since it is also the definitive reference for templates in general.