Secure Xml
SecureXml: The New Syntax for Signatures and Encryption, ISBN:0201756056
This book is written by DonaldEastlake, chairman of the IETF/W3C XML Digital Signature working group. The writing style is said to be terse but is also comprehensive.
It goes into heavy topics such as creating CanonicalXml.
See XmlSecurity for a book that is easier for beginners.