Pl/Python runtime overhead

Greetings,
Somehow I have failed to find the appropriate keywords for successful
results for my question.

When a pl/python based function is invoked, does it keep a python runtime
running across calls to same function? That is, if I use connection
pooling, can I save on the python runtime initialization and loading costs?

Are there any documents/books etc you'd recommend to get a good
understanding of extending postgres with languages like python? I'd really
like to get a good grip of the architecture of this type of extension, and
possibly attempt to introduce a language of my own choosing. The docs I've
seen so far are mostly too specific, making it a bit for hard for me to see
the forest from the trees.

Regards
Seref