The key features to look for in a scanner are image quality, software and ease of use. High end scanners will have faster scanning speeds, full-feature software and professional quality image processing. Basically, a scanner reads the data contained in a picture or document and converts it into digital form which you can save as a file. It takes between 8 to 20 seconds to scan a page. Sheetfeed scanners have limited value since you can't copy pages in books conveniently.
Drum scanners are relatively expensive.
Files resulting from high resolution drum scans are relatively very large. I had a service bureau do a drum scan of a transparency loaned by the Getty Museum and it took up 6M on a Zip disk. Flatbed scanners have the most to offer at an affordable price. A flatbed scanner makes it easy to scan photographs, letters, documents, pictures and pages in books. Please remember to observe the copyright laws when it comes to acquiring images from books and other print media.