One of the hardest parts of reviewing books (both privately, and in a public forum) is in justifying one rating against another. In my own rating system, a colour-coded spreadsheet which my wife rightly points out is somewhat over the top, I allow for half-ratings (I readily admit this is something of a cop-out, as it allows me to pile on the 4.5 ratings while preserving five star ratings for the best of the best. Should I start allowing for quarter stars, or is that going too far?), which helps but doesn’t solve this problem.
For example, I recently gave both Robert Greene’s Mastery and Joan Didion’s The Year of Magical Thinking a 4.5. Do I personally believe both books to be equal in status? No. For me, Mastery is the superior book (no offence Joan), but when I rate books I’m rating them against similar books in their field, as well as against other titles by the same author. This means that Mastery was held to a higher standard than The Year of Magical Thinking, in deference to Greene’s previous books, which I believe to be superior works of his.
I’m sure this is a wholly unfair strategy, but it works for me.