When doing archival research in Japan, I had a Canon flatbed USB scanner that could fit in my backpack and ran off USB power. I believe they call the series the LIDE scans. They're quite nice and very cheap, less than $60 or Y7,000. The problem though is that they are rather slow. They also do not have a lot of depth of field, so you really have to PUSH the book onto the bed of the scanner in order to read the text near the spine.
I found that if I was trying to copy a lot of pages, it was faster to set up my Canon 10D on a tripod (Velbon Carmagne) and photograph the pages instead. With the flatbed, I could maybe scan one page a minute, with my 10D, I could photograph over 10 pages a minute. At 6 megapixels, this is just about the same as scanning at 250 dpi. It was also easier to photograph fragile material like rare books, without breaking their spines by forcing them on the scanner.
With a remote shutter release and tripod, I could safely shoot at 1/6th of a second even with an aperture of f/8 to ensure good sharpness. The slow shutter speed meant I could use the ambient fluorescent lighting in the archives I was in, so I didn't have to set up my flash units, which also alleviated the blood pressure of the librarians whose patience I was taxing. It also meant I didn't have to worry about harsh shadows or reflections from the flash units. Soft ambient lighting is always your friend if you don't have to worry about handholding the camera. If you don't have a remote release, the 2-second self-timer on many cameras also works well.
You need a tripod where you can reverse the center column so that you can photograph with the camera upside down. Cheap $50 video tripods don't have this feature. You need to get a $150-200 tripod designed specifically for photography -- Gitzos, Velbons, and Manfrotto all make excellent photography tripods. A good ballhead will also make your life easier. My favorite field head is the Acratech Ultimate. More reviews of tripods and heads on my main site.
Lock up the mirror if you can (mirror pre-release) and use manual exposure and manual white-balance rather than automatic. This will ensure all pages of your document are photographed equally. You don't want one page to be yellow and one to be light grey due to the presence or absence of images, for example.
If you have some drafting/masking tape, it helps to tape the outlines of the frame of view onto the desk or whatever you're shooting on. That way, you don't have to look through the viewfinder each time. To prevent stray light from affecting the photo, be sure to cover the camera eyepiece with either the eyepiece cover or something that will block light (I find film canister caps work well, but a CF card should also do the trick).