This is a mix of a Free Book post and Reading Tools post. When I first started reading a lot of electronic books I had two main sources. One was Project Gutenberg and Christian Classic Ethereal Library for public domain books. The other source was Baen Publishers.
Baen is a Science Fiction and Fantasy publisher that has been around for a couple decades. I first found Baen when I was told about their free library. Baen started giving away free ebooks just short of a decade ago and now have more than 100 free books. Many of them are early books in a series. I explored many new authors this way and have ended up reading probably close to 200 Baen books over the past decade.
In addition to the regular free library, Baen has CDs that they put some of their hardcover books. These CDs have extras like artwork, some audiobooks and even more free ebooks. The great thing about the CDs is that several series of books have the whole line on the CD. The CDs have been creative commons lisensed so you can give them away as long as you do not sell them.
One kind webhost has compiled the now close to 30 CDs (lots of duplicate content) on his website so you can just go browse the CDs virtually and download what you want. The site is called The Fifth Imperium.
If you have any interest in science fiction or fantasy this is a great site.
Here are some suggestions:
Honor Harrington – Military Science Fiction – big epic space battles – 13 books free so far.
Legacy of Aldenata – Another military science fiction but based in the near future when the Earth is invaded. First four books are very good, rest of the series is very mixed.
Empire of Man – This is more of a fantasy than science fiction series. Traditional coming of age story with a spoiled prince that becomes a real leader over the course of four books. (Look for the Empire of Man section.)
Council Wars – This is a fantasy/science fiction mix of a future Earth that had achieve a utopia status and then was taken down by an evil plot. Virtually all technology was lost and there is a fight to regain control over the Earth. (Look for the Council Wars section on the right.)
Paladin of Shadows – This is modern military/terrorism series. The writing is good, but it gets sexually explicit and I stopped reading after a couple books.
There are another half dozen series of books on these CDs that I have not read yet. As you can see, the focus is mostly on military science fiction, but there are lots of good books.