There are some weird answers in this thread. Anyways.. my advice is to "interview" coaches via email before signing up with one. Easy as that. I have found a few just from browsing online, or people who post on Instagram, etc. I emailed all of them, explain what I am looking for--a training plan, interaction, explanations as to why workouts are in certain places, etc. Some reply with detailed responses, others don't, and that's how you weed some people out.
Reading one book won't necessarily tell you the "why". Different coaches have different theories. So yes, you are right in wanting to know why and actually understand what you are doing out there. Some people just get a plan and do it, no questions asked. Others, like myself, want details as to the theory behind it, especially if you are looking to improve in certain areas. Anyways, email interviews are the best way.. or even phone calls.
Nate Jenkins with Lowell Running Co. is great at giving you all the information you need, for example. He will tell you exactly why you are doing everything.