Here they are!
www.mobipocket.com/en/downloadsoft/default.asp
www.pocketpcfreewares.com/en/index.php?soft=595
You should already have microsoft reader, it comes with your pda.
I like mobipocket, however I'm trying out the free version of u-book now and I like it a lot so far. The latest version of u-book works in VGA (I think), but I don't have an X50 so I don't need that. You can check that version out at their website. Just do a google search and you'll find it easily.
I hope this helps. :)