When I needed to learn XPath and XQuery I went to W3Schools (www.w3schools.com), the tutorial was straight forward and has a good reference section.

I started off as a VB6/VBA programmer years ago with the Microsoft Press book Visual Basic Step-By-Step and added SQL to this later.

CodeAcademy (http://www.codecademy.com) has a basic introduction to Java.

However most of the time I learn best from books (not e-books) and have found the O'Reilly books some of the best references with their "In A Nutshell" range. I also have a large collection of Microsoft Press books for topics such as ADO.NET, Powershell and C#.