The ODE Manual is separated in the chapters shown on the table of contents on the right. The best place to start is the introduction.
This wiki supports the Collection extension: just use the "Create a book" link on the left, collect the pages you want, and download the PDF or order a printed version from PediaPress.
You can also access the whole manual in one single page to quickly search for function names.
Here is a short implementors guide for creating new joints in ODE (PDF format).