Keith,
Are you familiar with a site named
http://www.chromasia.com
He does tutorials that are a fair price, and he actually did 2 tutorials on my work with textures that goes in depth in to my work with my examples and how I did it. Here is a more direct link.
http://www.chromasia.com/tutorials/online/wwt1_info.php
It is a two part tutorial using my work and his work. However, if you don't want to get involved with that, just keep playing with different images and try different blend modes on the texture and opacity changes as well as masks to alter part of the textures. I sometimes use multiple textures on top of each other at different strengths. It's not uncommon to end up with 10 or more layers and huge files.
The chromasia tutorial would be better than a book. I don't make any money from this by the way, so I'm not trying to push his tutorials. He does do a great job of teaching though.
Hope this helps
mike