With an extra Dll file that allow windows Mobile apps to run on WinCE (Windows Mobile is based on WinCE), µBook Reader is compatible too, and will adapt itseft perfectly to the 800x600 resolution (I used it on a 800x480 5'' WinCE Nexio Tablet).
Adobe Reader for Pocket PC 1.0 works on WinCE too (not the 2.0 version though) and will use the 800x600 resolution. Slovoed Dictionnary works too.
Too bad Windows Mobile haven't been chosen over WinCe, even if the license cost more. Windows Mobile is much more user friendly and a lots of great ebook apps exists for it, like tomeRaider, Repligo, various Dictionnary, etc. Only a few of them will works on WinCE. Some will works but only use the standard 320x240 WinMobile Resolution.
Still, if the actual hardware is fine, it should be a nice device open to many formats