You can find a PDF version of the Linux DVB API version 3 in the download area.
A browsable online version of the Linux DVB API version 3 documentation (generated with latex2html from TeX sources in the CVS) is here.
A small collection of READMEs and FAQs is included in the Linux kernel source tree in linux/Documentation/dvb/ ( browse docs in the mercurial repository).
You can find a PDF version of Video4Linux2 API at download area.
A browsable online version of Video4Linux2 API is at here.
For developers, LWN.net is writing a V4L2 series of articles focusing at the development of a Linux V4L2 driver.
There's a new V4L Drivers guide at
here.
PS.: This is a newer document and might be suject to some errors. Contributions are wanted. Also notice that the main guide shall always be V4L API2.
Video4Linux API version 1 is obsolete. Driver and application
developers are requested to migrate their work to V4L2 API.
There's an effort in course to remove its support from
the drivers, keeping just a conversion module for keeping
compatibility for the old binary-only V4L1 applications.