Bullet User Manual and API documentation
From Physics Simulation Wiki
Bullet is a Collision Detection and Rigid Body Dynamics Library. The Library is Open Source and free for commercial use, under the zlib [LICENSE].
Bullet User Manual
A good starting point is reading the Bullet User Manual, the pdf file is located in the root of the Bullet Physics SDK.
You can generate API documentation using doxygen.
There is outdated doxygen-generated API documentation online:
- browsed online