Scanning books or their dust jackets? I imagine the scanner lid won't close properly with a book under it, in most types of scanners. One option: Photograph the books with a DSLR or other digital camera, and use a program to auto-crop them. This way you could even crop two or more books at a time, speeding up things.
Programs for image cropping and document cropping are different. The former detect images' edges and discard the scan's background; the latter detect page edges and crop along those. OP wants to crop scans of book covers, which are akin to images - so an image scanner is needed.
Also, if you have multiple scans, each containing one or more images, it'd be nicer to crop them all with one click. In Photoshop, you could do this by doing File > Automate > Crop & straighten on the whole batch, with a script like this. If you're not au fait with scripts, Snip app for Mac does the same thing (There's another similarly named app, SnipTag, for automatically batch-cropping scans and editing image metadata, but that's outwith what OP asked.)