The ARRL Handbooks provide a measure of reference information... some of
the vintage handbooks are better at describing AM which was most popular
in the 50's through the 70's.


Although they teach AM wrong, like the question on the amateur exam.

For AM look he
http://www.cbtricks.com/~kd6tas/amsig.htm

This is the correct information which can be verified by reading
chapter 4 in the RF Technician's Handbook, 2nd edition, by
Cotter W. Sayre, 621.38.