Just a consideration - the X5 has both compact flash and sd card slot. I think the X50 came out next and did NOT have the CF slot but built in wireless (correct me if I'm wrong). Then came the X51v which brought back the CF slot, SD slot, AND built in wifi/bt.
It is handy to have BOTH the compact flash slot and the sd card slot sometimes. For instance - I put all my programs and data on the sd card. If I want to play music or videos, those go on the CF card. I can pop that out and in without losing my programs. I long for built in wifi/bt but if I had to sacrifice - I don't really do much surfing on my axim.
And for Ebooks - look at the Mobipocket Pro reader with possibly the Office Companion. If there is a better reader out there I haven't seen it. With that and the clunky Microsoft Reader that some books still are published in

you'll be set.