I have been a recipe tester for several cookbook authors over the past three or four years. I have tested for individuals with a traditional book deal and for individuals self-publishing. I volunteer for recipe testing because I have aspirations to someday write a cookbook. Therefore recipe testing for others is a way for me to gain insight into the process.
I view recipe testing as a personal favor to the author. I was therefore surprised to have only once received a 'Thanks for testing' email. Most authors don't acknowledge receipt of my testing notes. I find this to be rude. None of these authors are super-famous (otherwise they would pay for testers). They are NOT too busy to send a quick 'Thanks for your feedback' email.
It appears that cookbook authors expect overwhelming positive reviews. One author had a two tiered system of testers and only those individuals that gave positive feedback continued to the second phrase of testing. One would think that in order to develop the best possible recipes a mix of positive and negative critiques would be required.
There is a certain amount of novelty associated with recipe testing. It's fun to be on the 'inside' of a cookbook, especially from a well-known author. If you are committed to the process of testing you will inevitably try recipes that you may have otherwise passed over.
Most authors insist on strict adherence to the original recipe. However, I would want you to use the recipe as you would if it was in a cookbook that you owned. Cooks are intuitive. If you don't have x, substitute with y. If you don't like x, substitute with y. If you like flavor/herb/spice y more than x switch it up. A good recipe needs to be fairly flexible. Recipes should still work with slight changes made.
When/if I ever employee recipe testers I plan on:
a. thanking each individual who did me the favor of testing my recipes
b. allow/encourage a degree of substitution (making a note of vital ingredients)
c. encourage constructive criticism
d. thanking each individual who testing for me - oh wait, did I say that already!?!?