Free books is an easy way to find and read classics. So much easier than checking out eBooks and downloading through library's Overdrive. The eReader app is slick and allows highlighting and notes.
Audiobooks were also easy to select and download. And they don't expire. The reader was OK, a little boring, but there is the option to "upgrade" to a higher quality for .99 cents. The file was large, 102 MB for an approx. 200 page book but they are easy to delete. I wouldn't want to save a lot on my device.
The above two would be good options for those times when someone needs a book for class or book club, and there is a waitlist, but they don't want to buy it.
iStoryBooks was so/so, limited and less professional.
I've used the YALSA Teen Book Finder in public library work during Teen Read Month. I used it to create a display of Teen Top Ten books that patrons could vote for.