This book is mostly an annotated version of the specification. For anyone seriously interested in XML, it is essential reading. Unfortunately, it appears to be out of print, but you should be able to find it in a library or get it from a second-hand bookseller.
This page contains links to all the important XML standards, including the current definition of XML itself. The documents are mostly dense and hard to read. For a definition of XML, you are better off with Graham and Quinn’s book. (See above.)