Each book is read and thoroughly tested by me in my home kitchen.
I aim to cook each recipe with ingredients as written. Occasionally, I have to substitute, or I might see a method running quickly out of control. In that case, I will note it.
The test includes a minimum of four recipes from each cookbook. For randomness-sake, I include the recipe on page 86 or whichever is as closest. Then, I aim to pick others in various sections to get a taste of the overall story. Why page 86? "Eighty-sixing" something is restaurant-speak for axing a dish or ingredient and most cookbooks seem to contain at least 86 pages.
Most of the time, I cook more than four recipes.
I choose cookbooks based on what is interesting to me, what I think readers will find interesting, and by publishing date.