The first thing you can do is take a razor blade to the inside covers. This will remove the hard cardboard covers (which you can scan with a flatbed scanner later) and leave you with the textblock. The textblock will either be composed of groups of pages called signatures, sewn together, or individual sheets of paper glued together, which is a type of binding called "perfect" binding. "Perfect" not as in "cannot be improved upon", but as in "completed, done, finished, the end" -- it is a very quick and cheap way of binding loose pages.
Anyway, the best thing to do is take two long pieces of wood (I recommend a hardwood such as oak, not pine or plywood), maybe a foot long, by a few inches wide, drill two holes on either end, get two long bolts and some wingnuts, and put them through. You've just created a bookbinding vise. Here is an example
of one which is fancier, which includes a squaring guide. BTW, the binding shown in that example is a very flawed perfect binding. A real perfect binding involves bookbinding glue and fanning the pages out a bit.
Anyway, stick the textblock into the vise so that the inside part of the textblock is sticking out by a millimeter or so. Make sure the textblock is approximately straight by measuring both sides of the stickout with a ruler.
Now get a sharp knife that can take a lot of pressure (a box cutter, utility knife, or some other thing) and start cutting off the inner part of the text block. It will take a while, and your knife will get dull fast, but you'll end up with really nice pages to feed to your sheetfed scanner.
Alternative methods involve table saws.