I'd recommend a Zodiac as an ebook reading device with it's excellent 480X320 landscape or portrait oriented display, dual SD slot expansion (one of which is SDIO) and huge internal 1540mAh Lithium Polymer battery for hours of ebook reading. I would recommend avoiding a device that takes AA or AAA batteries as they'll typically be much thicker and require using rechargable NiMH batteries or disposable alkaline batteries which would become cost prohibitive.
As a matter of fact, MAKE
magazine editor-in-chief and Zodiac owner Mark Frauenfelder writes about reading eBooks on a Palm device on page 134 of Volume 01
(great magazine), and he mentions the Zodiac as well as showing it in the article's main photo.
Click to enlarge
I was going to start a separate topic about the article, but it's pretty relevant to this thread too