Decide whether you need the books scanned so you can make changes to them or whether you only want to be able to preserve and read them. If they are books that you need to alter, you will need optical character recognition (OCR) software such as OmniPage. This type of software not only scans pages, but also "reads" them so the text is still text. This means it is editable with a word processor. If you only want to read and store books digitally, nearly any scan program will work.
Choose your method of digitizing. If you have old, rare and fragile books, you probably don't want to risk cracking the spines or damaging delicate pages, so flatbed scanners won't work for you. If you have new books to digitize or big coffee table books, flatbeds can work or a hand scanner might be the thing.
Use a flatbed scanner, such as those from Epson, Hewlitt-Packard Xerox or other manufacturers, for the clearest, best images for either OCR software or traditional scanning software. It takes some time because you need to place the book face down on the glass scanning surface, scan it, save the file and move on to the next page. It does produce the best scans, and you can change the resolution for greater quality.
Use a hand scanner, such as those from DocuPen, for delicate books. This is also time consuming, but it diminishes the potential for damage to the pages and the spine of the book. You put the book spine-down on a flat surface, open to page one and slowly pull the hand scanner down the page. There is a potential with this technique for slippage, meaning if you don't use a steady motion, the scanner can skip lines or bunch them into each other.
Use a digital camera. This is the easiest, quickest way to "scan" anything because essentially, you just point and shoot. There is some clean up work on the other end where you will need to straighten crooked pages and crop out the edges of the book or the background. This technique is best when the book can be held in a book holder with clamps to hold the pages flat. Be careful with older books because even the holder can damage a book. Also, this method generally will take images of the pages, but not allow text editing later. OmniPage does have a method to fix this.
Save the files, and do any fixing they need. If you are using image-scanning software, you can save the files as JPEGs, TIFFs or PDFs. Saving them as PDFs is the best method because if you have Adobe Acrobat, you can string all the pages together in order and make a readable and printable digital book. If you are using OCR software, you can save the files to word processors such as Microsoft Word or Pages. These, too, can be strung together as complete books.