I have made good experiences with my N800 as an Ebook/PDF reader. I usually read horizontally and scroll down, rather than toggling the display to fullscreen (too small then).
Contrary to what some others say, reading even large documents on the N800 remains comfortable as you can zoom to a size that is both clearly readable (nearly as in a written book) and easily scrollable (within the page).
I prefer the built-in PDF reader to Evince as it is more stable and easier in handling (I find). Good though that you have a choice. I once thought about a WM6 PDA and was surprised to see that no PDF reader at all (!) is running on that OS.
Scrolling and turning pages could be enhanced in both readers; it would be good if both operations were supported by buttons; now I have to go through the menus instead.

If your ebooks are available in HTML format, use that instead, because the browser breaks lines itself and you can comfortably read at any text size without scrolling sideways!