Book D is the name given to the specification of DVD-Recordable. The DVD Forum backs and promotes DVD-R (write once) and DVD-RW (rewriteable). A blank single-layer DVD-R disc will store 4.36GB of data (dual layer DVD-R's, or DVD-R DL, entered the market in 2004, and provide 8.5GB of storage).
Write speeds of up to 18x are available for the DVD-R format. DVD-R competes with DVD+R, a slightly more advanced forward backed by the DVD+RW Alliance.
There is also...
Book A (for physical DVD format specification)
Book B (for DVD-Video)
Book C (for DVD-Audio)
Book E (for DVD-RAM)