evilcelery has the best answer +1 from me;
Most anything you run across for npm will work just as he said, and lib is the best convention to do it with.
To expand on his links a bit The module list he refereed to is found:
https://github.com/joyent/node/wiki/modules
http://search.npmjs.org/ is a bit more user-friendly.
Also if you wish to include things globally similar to npm you can do what it does with things like html and put it in the lib dir where you originally compiled it with the Makefile (note: you may not need to rebuild it but I beleave you do)
There are a lot of mongodb related projects/libs enjoy!
interestingly:
Blockquote Contrary to the belief of many, "npm" is not in fact an abbreviation for "Node Package Manager". It is a recursive bacronymic abbreviation for "npm is not an acronym".
source: https://github.com/isaacs/npm/blob/master/doc/faq.md#readme