There's a series of books titled "Very Short Introduction" where many of them will cover schools, topics, and thinkers involved in philosophy. They run about 150-200 pages and often provide a good overview of the material. I read the books on philosophy, philosophy of science, Habermas, etc. and they were very good.

I would start with "Philosophy: A Very Short Introduction" by Edward Craig, see what catches your interest, and go from there.

Also +1 for Russell's "A History of Western Philosophy".