How to determine which college textbooks are most widespread?
This is not a literature review question; this is a market research question. (That's not a problem, but understanding the nature of the question helps to determine the most appropriate approach to take.)

Are you asking this question out of personal curiosity, or do you want to undertake a formal research study?


Personal curiosity: I recommend that you contact sales representatives of multiple publishers and ask them these questions. You should first contact the representative for the books with the contradictory definitions. You can ask each of them which are their main competing books, and why their book is superior. (If you only ask the main competitors, they might be reluctant to give you that information; however, if you ask them to compare themselves, they will likely willingly volunteer that information.) You can ask each sales rep your questions about the market penetration of their textbooks. I doubt looking at Amazon rankings would be meaningful, since I would guess that most university textbooks are still purchased through university bookstores, not through Amazon--I don't know that for a fact, though.
Formal research: You need to look into how to do a formal market research investigation. Sorry; I can't help you much here with practical details, but at least I hope this points you in the right direct.