OpenLayers
I've done a few OpenLayers project since starting this page as a place holder 2 years ago. Information about them (and more about OpenLayers) is scattered around the Wildsong web site.
What is OpenLayers?
OpenLayers (http://openlayers.org/) is a library of Javascript code that you download and put on your Web server. You reference it from your own HTML and Javascript based pages, to put maps into your web site.
Docs for programmers: http://dev.openlayers.org/docs/files/OpenLayers-js.html
OpenLayers is very stable code. It has been at version 2.x as long as I have used it. Version 3 has been a long time coming, we await anxiously.
Books
OpenLayers is so popular in fact that there are even books about it. And it is so stable that even a book release two years ago is very useful.
PacktPubs has a book called [https//www.packtpub.com/openlayers-2-1-javascript-web-mapping-library-beginners-guide/book OpenLayers 2.10: A Beginner's Guide]] and it is how I got started with OpenLayers.
The author Erik Hazzard has a page about his book is here: http://vasir.net/openlayers_book
I also have a copy of the new PacktPubs short book.