Last weekend this book fell into my hands:
Paul Lawson wrote a splendid introduction into Apache SOLR. At the beginning he describes the motivation behind the project SOLR:
"Outside the firewall [searching] is used to make money, and inside to save money."
Within 130 pages this books covers everything from installation, adminstration GUI, examples to start (importing documents, first searches, etc.) and the quick overview to the fundamental configuration files.
If you are interested in Apache SOLR and you do not know where to start: this book is the answer.
But one thing is missing: The book only covers searching XML documents. On page 42 (!) it is pointed out, that you have to configure SOLR cell. I think this was a good decision, because deep dives should be done after an introduction.
if you are interested, take a look at my review at amazon.de. (as all my reviews written in german ;-)