MOBIs are your best bet from FictionWise. An alternative is ePub from Books on Board. With modern MOBIs, there is probably little to choose between the two although images may be compressed and smaller in the MOBI (this is less of an issue that it used to be) and ePub has a richer set of formatting capabilities than MOBI. The eReader version would work well on a small handheld device, but its simple formatting and small images probably make it less suitable for dedicated reading devices.
For a look at all secure ebook formats, see
ScreenShots of The Hobbit.