I start with the top 100 sellers each time I look
http://www.amazon.com/gp/bestsellers/digital-text/ref=pd_ts_pg_1?ie=UTF8&pg=1
there will always be some free books in there (lots of classics).
The other way is once your in the store, select a section and sort low to high price. Some of it is really old stuff and you have to weed through, but there are lots of gems (I love Pride and Prejudice and have it for the ereader but just got it for my kindle).
I know some other free ebook sites as well (I love free ebooks) and while I would say 70% of it I am not really interested in, the 30% I do find that I use have been real gems.