As we speak, there are over 7 million books ranked on Amazon.com. There are probably just as many books that have never sold a single copy and are not ranked. How do you know what book to buy or what author to follow?
Here are a few hints to help:
1) Avoid authors who take themselves more seriously than their work.
If an author attacks their competition or talks excessively about themselves or how they are better than others, they take themselves more seriously than the books they write. The public deserves less ego and more research from those whose books they buy. (Ego kills the truth.)
2) Look for crusades and prejudice, which would interfere with or color the truth.
If an author has a lifestyle or obsession that may cause them to ignore or stress certain ideas one should not trust their opinions or interpretations.
3) Look at other books they have written to discover how trustworthy they are in their work.
Would you trust someone who places their own "bend", lifestyle, or preconceived ideas above the truth? For example, there is an author who claims to have "translated" the New Testament, claiming the Bible encourages a gay, lesbian, and bi lifestyle. Greek scholars have said "this interpretation so twists the language that it should be called torture." This shows the person begins with a point of view and renders texts to fit a personal view. This breaks the first rule of letting the text say what means without getting in the way.
4) Remember that education and degrees teach us how to learn. They do not teach us enough to be experts. That comes from extensive personal digging. To tout a degree over one's experience and study is a sign of arrogance and is an attempt to place oneself above others. I have known educated and arrogant idiot and brilliant laymen.
5) Listen to audios, videos, and webcasts of the author. Their presentation will tell you more than you can obtain through the written page.
6) (This one left me aghast.) Look at the book reviews of the author's competition. If there are low ratings left by the same people or group of people on 3 or more books of the competition the author that you are investigating is using unethical means to undermine others. If you buy a book by an author and you do not like it, you may give him or her a second chance, but you will not likely give him or her a third chance and you certainly would not give books 1 or 2 star rankings on many books because you would not buy books from an author you hated that much. Conversely, run away from authors who attempt to blame others for their bad reviews. If a book has a high number of reviews and a high percentage of negative reviews it is because the book is not good. However, remember that you must decide this over a number of reviews since the first dozen or so will be strictly family, friends, and enemies.
7) Throw away the first 6 to 10 five star ratings. They are usually from the author's friends and family. Throw away the first 3 to 5 lowest rankings. They are usually from enemies and competition. The rest are probably from the masses. Look for ratings coming from the same area. they are from followers of the author (again, usually friends or family). Look for the number of "Likes" on Amazon.com that are out of balance compared to the number of ratings. If they are not in like on a 1 to 1 or 2 to 1 maximum where "Likes" exceed reviews, the author is using unethical means to increase "Likes" themselves.
8) Look at the "Tags" on Amazon. If the book has been placed in an inappropriate category, and if several books of the author have been placed in odd categories, such as "public domain" or categories which are demeaning to the book, you can bet unethical competition or general malice is behind it.
9) Check out the "publication date" of the books of authors you are comparing. If there is a general pattern where books of similar titles are published by author "A" always after those of author "B" over several books you have a serious case of one riding the coattails of the other. Such actions are designed to impede the first author and expand the second by taking advantage of the publicity already in place. The publication of the second author is usually followed by a smear campaign against the first. Avoid that author at all cost.
Look at the people ranking and rating the books. If they are authors you can bet there is "cross ranking going on where authors give good grades to the other authors books in exchange.
For all the above reasons, plus the fact that Amazon, Goodreads, and other book rating sites openly allows this type of behavior, ranking and ratings on Amazon, Goodreads, and other book rating sites cannot and should not be trusted. This I say as an author with several books in the top 10 of various genres. Because of the passions evoked by my books in the religious community, we stay under attack, yet most can be traced by to other authors and his or her friends. Still, we have many books in the leading slots of various categories.
You cannot take these things too seriously. The world is full of fools and thieves. Write out of passion, not to win. Winning will naturally follow the fact that you enjoy and have passion for your work.
There is much more information to convey and many other tactics I am yet to experience. All of those I have experienced so far have taken me by surprise. When I began writing, many years ago, I thought "scholars" and writers had integrity. As it turns out, many do not, and the worse offenders are writing in the field of religious books. I am sure many niche markets have hucksters also. Go forth with passion and you will conquer.
UPDATE - Nov. 2011
After writing to Goodreads and alerting them to the constant one star ratings from our competition they investigated and confirmed that it was indeed a personal attack. Goodreads removed the reviews, which all came from the same person. This was the second such attack and involved over a dozen titles.
After lodging several complaints about the same issue, Amazon.com has refused to act and has permitted one star reviews to be posted on many books saying everyone has a right to review as they wish, even those who are not moral or ethical.