If you're just getting into Trek lit, a decent idea might be to pick up a few of the numbered novels and see how you like them. The numbered novels are different from most of the current Trek lit, in that each one is pretty much a stand-alone affair. The downside to the numbered novels is that none of them do anything to contradict or further events that happened in the series' proper. Take TNG novels, for instance. It's usually, there's a crisis of some sort and the crew solve the crisis in 280 pages or less. Actually, that's more or less true for most of the series numbered novels. For this reason, a lot of people find many of the numbered novels boring. Personally, I don't. They're fun in their own way -- kinda like a stand-alone episode you've never seen playing in your head.
A good place to start looking would be Ebay. Search for 'Star Trek Lot' in the books category, and you'll find dozens of lots of Trek books cheap. Opinions vary greatly on which numbered novels are good and which aren't, but the advantage to buying a lot is that, even if you get a couple that didn't thrill you, hopefully, you also got a couple that did. In any event, you're probably not out much money. I've bought most of my Trek books from Ebay, and I now have over 500.