Depends on how technical you want it. What's your background like? Med, Comp Sci, Eng, hobby? Most of the good stuff is in computer science or neurology books. Comp sci mostly focuses more on function, and neurology mostly focuses on form.

I think one of the best approaches from the ground up is in Stephen Wolfram's "A New Kind of Science," which can be found at

http://www.wolframscience.com/nksonline/toc.html

It covers the theory behind emergence in terms of finite state machines and shows how a series of simple rules often lead to complex systems.

I'm currently writing my honours thesis on this subject. I'd suggest "Emergence: Contemporary readings in philosophy and science" edited by Mark Bedau and Paul Humphreys. "The Re-Emergence of Emergence:The Emergentist Hypothesis from Science to Religion" Edited by Paul Davies and Philip Clayton. "Emergence or Reduction?: Essays on the Prospects of Nonreductive Physicalism" by Ansgar Beckermann and Hans Flohr. Jaegwon Kim is a notable philosopher on the subject and Stuart Kauffman has written extensively on the subject so look for Kauffman's new book "Reinventing the Sacred: A New View of Science, Reason and Religion". I saw a lecture he gave about the book and he talks extensively about ontological emergence in it.