Aldiko is a class act when you get it going. There are a zillion options to set the font type/size, line spacing, margins and colors (try non black and white) to make reading even easier than physical books. But it can sometimes be tricky to get your book data into aldiko.

Another app called overdrive has almost as many options for readibility. I discovered by accident that if you go into manybooks.net in your tablet web browser and download one of their zillion of free books, it will ask you whether to hook up with overdrive with epub and load it directly in! It's just so slick and you find results right away in overdrive.

Furthermore overdrive will check books out of many local libraries including audiobooks! It's advertised purpose is more along these lines, where it will make the book "vanish" when return time is due.