One subject that needs to be dealt with is the minimum focus distance (MFD from now) when you zoom.
I haven't thought of it as a problem until yesterday but it seems be a growing problem. My old Canon PowerShot A540 in macro mode have a MDF of 5 cm (2 inches) when not zoomed (W) and 30 cm (12 inches) when zoomed (T). In my experience the 30 cm kicks in as soon as you zoom, not just when you're at full zoom. Without macro mode the MFD is 45 cm (18 inches) for both W and T.
For the newest line of Canon PowerShot A-series cameras that has changed quite a bit. It might have done that long ago but I haven't noticed before.
The Canon PowerShot A810 can serve as an example since it seems to be same for all in the new line. The specifications are ( http://www.usa.canon.com/cusa/support/consumer/digital_cameras/powershot_a_series/powershot_a810#Specifications
Normal: 2.0 in. (5cm) - infinity (W), 3.9 ft
. (1.2m) - infinity (T)
Auto: 1.2 in. (3cm) - infinity (W), 3.9 ft.
(1.2m) - infinity (T)
Macro: 1.2 in. - 2.0 ft. (3-60cm) (W
It has to be confirmed but this could mean that if you're book scanning you can't zoom in unless the camera is at least 1.2 m from the platen. Basicly the same seems to be true of a camera like Sony DSC-W570.
So it seems MFD when zoomed is something to look out for. If you can't zoom you might need to able to change the distance between camera and platen more to adjust to different sized books (or live with wasted pixels and lower DPI). And if barrel distortion is a problem when the camera isn't zoomed the only way around might be to build a scanner were the cameras are something like 3-4 feet from the platen.