I'm not quite sure what you're aiming for, but the Be Book is available online (https://www.haiku-os.org/legacy-docs/bebook/), and includes all the documentation for the API. I don't know that any of the internals stuff is online, though.

EDIT: Haiku also has the Be newsletters (https://www.haiku-os.org/legacy-docs/benewsletter/index.html) and "Programming the Be Operating System" (https://www.haiku-os.org/legacy-docs/programming_the_be_oper...), which actually does encompass everything I think you were looking for.