I find Jan Axelson's books fairly good but his mostly focus on PC standards (parallel port, serial, USB, etc.). He even has a book on 8052-BASIC.

His web site has a good deal of online material under the resources section.

http://www.lvr.com

Another source I find useful is a web site called Webopedia. Type in a key word and there are usually plenty of helpful links on the desired subject.

http://www.webopedia.com