Book E is the name given to the specification of DVD-RAM. DVD-RAM stands for DVD Random Access Memory, a disputed name. Nevertheless, it does provide random access. DVD-RAM discs can store about 4.3GB of data on a single side and has progressed very well in recent years. Write speeds up to 16X are supported. These discs are marketed as being re-writeable up to about 10,000 times.
The DVD-RAM format is not fit for use with DVD-Video for example, yet DVD-RAM is a suitable format for camcorders and DVD recorders. It offers a form of Defect Management system for data protection also. Linux and Mac support DVD-RAM directly also. The format is currently being pushed by the DVD-RAM Promotions Group (RAMPRG).
There is also...
Book A (for physical DVD format specification)
Book B (for DVD-Video)
Book C (for DVD-Audio)
Book D (for DVD-R)