Trying to understand SPI

Greetings,

    I have recently started working on PostgreSQL and am enjoying some
of its exciting features. I would now like to understand more about
incorporating my own functions into the Posgres backend. I have read
what I believe to be relevant documentation, and I have several questions.
Unfortunately a search of the mailing list archives could not yield a
"hit" on the phrases that I could come up with.

1) When I tried to compile some functions of my own, many header files
   could not be located. I therefore copied the entire src/include tree
   into /usr/local/pgsql/include. Is this a Bad Thing?

2) The examples in contrib/spi use several functions (elog, SPI_execp) and
   typedefs (Relation, Trigger, Datum, EPlan) , which are not documented
   in the Programmer's Manual. Where might I find the relevant
   documentation?

3) Does one need a good grasp of PostgreSQL source internals before one
   can use the SPI and other Extensible features of Postgres?

Thank you for time and attention.

Gurunandan