Are you looking for a comprehensive XML book?

I've read major parts of the following book and it helped me understand XML much better:

HISTORY OF XML XML is short for eXtensible Markup Language. Both XML and HTML (Hypertext Markup Language) are based on SGML (Standard Generalized Markup Language). The SGML specification was written in 1974 and became an ISO standard in 1986. At that time there was still no internet. In 1998, the World Wide Web Consortium - http://www.w3.org - made its first XML recommendation.

HTML - WHERE IT ALL STARTED The HTML language was initially developed for describing the layout of documents. While browsing the internet, commonly the browser is displaying a HTML document, which contains the text the browser needs to display and information on how the browser should display it. Later on, cascading style sheets or CSS were developed in order to separate the textual content from its presentation. Typically, a HTML document references a CSS document. A browser intelligently merges the content and how it should be displayed.

HTML documents are normally edited with an HTML editor.

In Microsoft Internet Explorer it’s easy to view the HTML structure of a webpage by right-clicking with the mouse on the page (not on a graphic) and select ‘View Source’.

HTML is less suitable for sharing and describing data in a standardized way, and that’s why the XML language was developed.

WHAT IS XML XML documents describe data in a structured way. Data in XML documents is typically hierarchically nested, which means that some ‘parent’ data can contain ‘child’ data. Each element should be closed and elements should be properly nested.

Unlike HTML documents, XML documents are difficult to read in a browser. Typically, XML documents are processed by a software program (called a XML parser), which extracts the data from the document.

In order for data in XML documents to be displayed properly in a browser, the eXtensible Stylesheet Language or XSL was developed. Typically, a XML document references a XSL document. The XSL document contains information about how the XML data should be displayed when the XML document is requested by a browser.

Further, XML documents can be validated against a so called Document Type Definition or short ‘DTD’. By validating a XML document, the well-formed-ness of the XML document can be verified. Typically, a XML document references an DTD document. The DTD document describes the structure of the XML document. XML parsers can validate XML documents with the help of the DTD document.

The following sites provide excellent, in dept information about XML:

A very good HTML editor is Adobe Dreamweaver CS3 (formerly Macromedia Dreamweaver 8).

A very good XML editor is Altova's XMLSpy.