Try www.rockfordfosgate.com for a basic start. www.jlaudio.com/tutorials is also very good.

Sums at www.mobileaudio.com/rac-faq

That last one is an excellent site.

There is also a lot of information on this site under Articles.

The actual sums are difficult to understand for some, so here's my basic explanation.

For series wiring, add the values of the coils together.

For parallel, divide the value of the coils by the number of coils.

Series-parallel is a mixture of both wiring methods as you probably know...