Thanks for that tip nitrexoxide. I have decided to take the plunge and go all out for the GIMP.
I have purchased Akkana Peck's excellent book, Beginning GIMP - from Novice to Professional. It is a brilliant book - 500 pages packed with excellent coloured illustrations.
Here is a link to the authors site:
If you have the dosh to spare and want to master the GIMP, then this book is the answer to your prayers - I know it's the answer to mine for a decent up-to-date book on the GIMP.
From what I've read so far, a layer is just like an OHP transparency. As you build up an image on the OHP projector using several OHP transparencies, you can do the same with layers in the GIMP. You just overlay different parts of an image using transparent layers, until you have the finished image. Then you can save it to file as one complete picture.