As for light-hearted fiction books - there's plenty. I just finished reading all 14 books in the Wizard of Oz series to my son (on my Nook). Zane Grey published a lot of Westerns as well as a few about baseball (I like the baseball ones). There's also some high quality public domain science fiction from authors such as:

Harry Harrison (only a few that he forgot to copyright) Cory Doctorow Peter Watts (Blindsight is awesome, Starfish is good too)

And if you like any of these, Feedbooks always suggests related titles at the end of the book that you might also like. On Kindles, these are live links for one click downloads.

Frankly, I'm amazed that the quality and organization of Feedbooks is so much better than the Nook and Kindle stores. Obviously there are far fewer titles though.

Here is what I mean by better formatted:

Table of Contents always included

No funny characters scattered throughout the text

No missing paragraphs or words

Graphics on the same page as their captions

I found I could not count on these things for books in the Amazon or Barnes and Noble stores for low cost or free books. Even more expensive books often lacked table of contents (when one was appropriate - for books with chapters or short stories). More expensive books often lacked appropriate handling of graphics - Nook is bad, Kindle is worse.

Feedbooks almost always has table of contents when appropriate, and never has missing paragraphs, funny characters, etc. Though once (out of around 50 books) I encountered a chapter that was the duplicate of the previous chapter.

EDIT: formatting