book tapes

I have been checking the library for book taps, most are CDs now or DVDs. Found a few book tape I like but not sure of some of the termonology. I know analog will work in my cassette player, not sure about digtal or what the difference is. Here are two of the discription of two different cassettes.
5 sound cassettes (ca. 6 hr., 44 min.) : analog, Dolby processed. and
7 sound discs (ca. 8 hr., 26 min.) : digital ; 4 3/4 in. I have a digital player from The Blind organization and they are about 4:34 in in size and will hold 42 hours of copy. The discs and the 4:34 in dont seem to go together. Just confused, hope you can help.