This 3rd edition version of the XML Pocket Reference from O’Reilly packs pretty comprehensive coverage of XML, DTDs, W3C XML Schema (.xsd), RELAX NG regular (.rng) and compact (.rnc), as well as Schematron rules (.sch) all into one slim 171 page reference book.

Schematron is uniquely suited to checking that XML documents confirm to non-grammar based rules; something other schemas (e.g. DTD, XSD, and RELAX NG) cannot do. It gets covered in here.

RELAX NG is slightly more powerful than XSD and way simpler than XSD. What is the advantage of XSD (W3 XML Schema, you ask? Good question! Not much.

The combination of RELAX NG + Schematron is as superior to W3 XML Schema as it is to the DTD standard, which has been inherited from the aged SGML standard created in the 1960s.

Some reviewers of this book harp on its mysterious decision to include Schematron and RELAX NG. Guess what, they have both been ISO approved standards (crucial to the DSDL, as a matter of fact) for over a year, and they have both existed for around a half decade or so.

Note that it is just a little bit too thick to fit in my shirt pocket. It is amazing how much information on so many useful XML standards they packed in one little book.