Use wooden dowels. For sturdy, modern volumes only, especially bound periodicals. Such volumes have a space between the sewn, bound pages and the spine of the book cover. It's as if the book spine were hollow. Obviously, this does not apply to all, or even to most, books.
But when you have such volumes to scan, you can slide a dowel into the hollow spine to open the pages almost flat by flexing the spine into a gentle arc. You will need a range of dowel diameters, and you choose the dowel size to best open the pages evenly. This is so effective that such volumes can be scanned easily and accurately on a flat-bed scanner, such as a Xerox machine. This is a standard technique in libraries with many such bound periodicals.
For some books without hollow spines, such as some hardcover books and some thick perfect-bound softcovers, you can place a dowel below the spine on a scanner that scans from above. This has a similar effect as placing the dowel in the hollow spine, flexing the book gently into an arc that can better be photographed.