I had Microsoft Reader on my Pocket PC but it's a pain. MS Reader has to have an external freeware application to convert text files to books, and not much control over the output. Also, lots of clicks to get from one book to another.
(As I say that, I realize how spoiled I sound-- remember our ancestors who came across the prairie, and didn't have any books except the ones they ordered by mail to be shipped to them around the Horn, or from traveling peddlars who didn't exactly offer a wide selection. But still, once they had books, they were able to have two or more open at the same time...)
Mobi Reader is a LOT better. No application to develop e-books and then install them. You just open a document or text file, it takes it from there and sends the book to the mobile device. Love it. Since most of what I like to read is so old that it's available for free on the internet anyway... I am set up for summer reading!