I have a Palm IIIxe. It's gray-scale only, and holds 8MB. I have both the Palm Reader and Acrobat Reader installed, with plenty of space for books.
I certainly prefer the Palm Reader but the PDF reader is usable. There are lags between pages--probably a newer and higher horsepower Palm wouldn't have these problems.
I bought my Palm a couple of years ago at Fries for $50 (refurbished). I've seen them on eBay for less than $20.
By the way, the IIIxe uses AAA batteries. This is actually good news since you don't have to worry about buying a machine which can't hold a charge. I bought a set of 4 rechargable AAA batteries and a charger for $10. A charge lasts a good long while--unless you use the back lighting.