It is easy to use. No one needs a training class on how to use a book. Sets of books can be easily shared among several users, which may not be the case with CD-ROM sources. Books never give the user error messages, and there is no fear of the network going down while one is reading. There are some disadvantages to the book, though.
Books consume a tremendous amount of space.
They are also labor intensive - they need pocket parts and filing updates, and they need to be shelved or shifted on a regular basis. There is no standard search engine, so the learning curve is steep. They are difficult to network, despite what some vendors will have you believe, and reading treatises online is often very difficult, especially with footnotes. These services also have some definite advantages. They are easily accessible from remote locations.