One possibility I'm looking at for book scanning is HP's old Scanjet 4670, which seems to have been VERY strange but would possibly do quite a lot of what I would want. Basically, it's a transparent inverted flatbed, basically a thick lid containing the scanning mechanism - you put things under the lid face up, you can see the alignment precisely because the scanner is transparent, glass front and back. Doesn't solve all of the problems of scanning old books etc. but it looks like it would generally get the alignment right first time, and re-positioning stuff repeatedly to fix alignment problems is one of the things that buggers books.
They don't seem to have a stellar reputation for colour fidelity, but since 99% of what I scan is grey scale or text that wouldn't necessarily be a huge problem.
The trouble is I've never used one, or seen one outside a photo, and can't currently find anyone with one for sale. Looks to have been USB, not USB 2, which may mean slowish speed. Anyone know anything about them? Was there a later version?