Are you serious about audio measurements? Then the “Audio measurement handbook” by Bob Metzler is a very good introduction to the topic. You can buy used versions of the book on Amazon for more than 200US$ or you can get it for free from Audio Precision. Check out their starter kit. The book is from 1993, however most of the basic measurements haven’t changed. There is only one topic that the book does not cover: measurements of Class-D amplifiers. You can find a white paper about this in the AP white paper archive.

BTW: I will test an APx525 in the next time and compare it to older audio test equipment. Whit a price of >10k$ this is definitely not a hobbyist instrument. It is todays gold standard for audio measurements.