My take on all of this is that if you read everything on any subject matter, you gain a lot of knowledge and after a while you are able to sift the good from the bad very quickly yourself.
I can read the first paragraph of a book and decide if its going to be any good or not. The problem with Russian history is that it appeals to a very small number of people, so even the best bookshops are not flooded with stuff.
If you have a wish list on Amazon, read the critiques written by people who have read the book, (you have to scroll down). Sometimes this helps.
Just like any subject, there is a lot of rubbish written about Russian history. Yes, books are very expensive, we have one of those 'chain' bookstores here with a coffee shop attached, and although I deplore the very thought of reading an expensive book while eating or drinking, they allow you to take books off the shelf, and 'peruse' them at the table. So I sometimes sit for hours with a pile of books and a cup of coffee, and decide for myself which of the books I cant live without !! My advice to you is to find one of them.
There was a time ( before the global recession ) when I was able to just pile them up and just pay.
Alas no more.
I hate Kitty Kelly by the way, God forbid she ever tries to write a book about the Russian Imperial family. I dont think she will, not contentious enough for her. Greg King and Penny Wilson I always find very credible, because their books are backed by archived documentation, original diaries etc. Sometimes the index is just as fascinating to read as the book itself. I think they work very hard before unleashing anything on the general public. In my opinion, world experts on the subject and passionate about the quality of the research they do. Radzinsky, too many personal observations and opinions which leave the reader with a confused version of the facts. I think however that his books are a must because he has access to archives which I don't think the Russian authorities are too keen to make available to anyone else.