Or under linux you can mount the iso and it will act like any other drive
mount -o loop -t iso9660 isoname.iso /mnt/Iso
They contain a lot of things, all are the books are offered in many formats as well as extras like cover art, animations and other goodies. For example the
Storm From the Shadows CD contains 46 books each in 6 formats along with the extras.
Here's a graphic breakdown of what is on the cd. Couldn't get the whole graph on the screen at once but most of it is there.