hi,
I have written a short introduction into Ambisonics, which lives at
http://www.sbox.tugraz.at/home/f/floholl/HOA_intro.pdf
The not-so short introduction is in my thesis, which is available at
http://www.sbox.tugraz.at/home/f/floholl/thesis_letter.pdf
The references at the end of the thesis might also help.
However, both papers are primarily concerned with the 3D case.
best, flo.H