First and foremost, any reviewer should always limit themselves to what they can do instead of taking on a whole industry. Once they are comfortable, they should branch out. When I first started reviewing books, I limited myself to book just on the Battle of Gettysburg knowing there to be enough information out there for me to continue that practice for some time. The more comfortable I became with the subject, the more I started to branch out. Now, my readers will notice that I review any book on the American Civil War. However, I do not believe that I will ever branch outside of the conflict to envelop all American Military History. There is just too much out there for me to handle when it comes to the enormity of American History. While Civil War publications are many, there are even more World War II publications to which my knowledge does not stretch.
This brings me to my next point about being a reviewer.
Second, you should only review that which you know about. Your job as a reviewer is important to the people, especially in this age of social media. There are those who trust you and follow you hanging on your every recommendation. Some people are even excited that a certain book will be coming out and wonder what it could be that you are going to say. They may not even like what you have to say, but in the end, you have created bond of trust. The most important thing is to not review something you know nothing about because your opinion may not be accurate since you know very little about it. Once trust between yourself and the readers has been broken, it is very difficult to get it back.
Third: Never always give positive reviews. This is a difficult one to say due to the ideals of the reviewer. You want to like everything you read when it comes to books and hope that someday when you write a book, the people whom you gave good reviews to will return the favor. This is not always the case. Readers will also notice if you, as a reviewer, always give good reviews. Eventually, they will lose their trust in your writing and stop following you. Readers may throw out terms such as "biased" or "subjective" but you always have to back up your claims with reasons for why you said things. This is what can happen if you only publish positive reviews. If you only publish negative reviews, no one will approach you to review your work; there would be no point if all you are going to do is bash a work. If you want to be a good book reviewer, especially in the non-fiction realm, than you need to find a happy medium between the two.
Fourth: Be ready to receive harsh words from authors and agents upon negative reviews. Usually, this does not happen, but there have been times when not only the author has lashed out, but the agent has had a few words as well. As a reviewer, it is your job to tell the public when a poor product has been released according to your opinion. If they do not like your opinion, they can find another who can review the work in a positive light. The ideal behind negative reviews is not to bash another person, but warn possible customers of the work that there are points in this work which just do not make sense.