Section 2) of this paper also has a very gentle and readable
introduction to the gc :

http://portal.acm.org/citation.cfm?id=141130

Another thing you need to know, if you have long running pieces of C
code that don't interact with ocaml's runtime system, is the two
functions :

caml_enter_blocking_section
caml_leave_blocking_section

They are explained in this message  :

http://caml.inria.fr/pub/ml-archives/caml-list/2001/06/58d7a7e8747056c3842e53b4e9454f44.en.html

Best,

Daniel