Ratings at Zubon Book Reviews come down to two binary questions: is this book worth reading, and if so, is it worth reading more than once?
The first question is fundamentally the reason why this site exists. After reading book x, would I recommend it to someone else? Each book is judged by the standard it sets for itself. You expect different things from academic non-fiction, hard science fiction, or epic romance. You are judged according to what you are (or seem to be) trying to do, rather than what someone else might have wished the book to be.
There is also a benefit-cost analysis incorporated in there. If you write a thousand-page book, it better be worth one thousand pages of reading. If you only have four hundred pages of content, you will probably get savaged on review. If it is difficult to read, you need to bring more to the table. If you have a picture book with ten words on a page, you need not write the Great American Novel to make it worth reading that book. Good fast food that costs $1 gets rated higher than expensive but mediocre cuisine.
The second question is whether I would read the book again. Life is short, and more text is written each year than you could read in a lifetime, so being worth multiple reads is something special. Also, if you are only going to use something once, there is no need to own it. Rent or borrow it. This is why we have libraries, second-hand book shops, movie rental stores, NetFlix, whatever. I only rate a book worth buying if I plan to read it more than once.
I note this partly as an apology to authors, since a great many books that I like do not get the BUY IT stamp. It is a hard standard to meet, but most people are not going to have as many bookshelves as I do. If I could get everyone to read one book a month, I would be giddy. If I were sane, I would recommend only buying a book if I plan to re-read it frequently, since I can still use the library if I need the book one week in five years.
So, the actual ratings:
4: worth reading multiple times (buy it)
3: worth reading once (borrow it from a library)
2: not worth reading (skip it)
1: not worth considering (burn it)
If you are a librarian, a 3 still means "buy it," since everyone in your community should read the book. Everyone, so get at least one copy and start recommending it, if you please.
A rating of 1 is something special, more rare than a 4. This goes beyond not worth reading. A 1 means that the book makes the world a worse place. Appalling writing and/or illustrations may get you there. The most common route is going to be non-fiction with disinformation. Disagreement or mistakes are not enough; a 1 means that you might know less about an issue after reading the book than when you started. A 1 on a fiction book means that the book brings negative enjoyment: not just "not worth it," but actual suffering (this may be the case for 1-rated non-fiction as well). Yes, I am aware that some things I rate as 1s are on college syllabi. This is a failing of the professoriate, not the ratings system.
We occasionally have half-ratings. These mean "in part," when part of the book merits one rating and part merits another. If the book has severable pieces, I can recommend just those pieces, and it gets the *.5 rating; if the sections cannot stand on their own, the rating probably gets rounded down, unless the parts worth reading are excellent enough to pull the entire book up. Books of essays or short stories are good opportunities for *.5 ratings.
Some books get starred ratings or notes that the rating only applies if this is your field of interest. Some picture books really are good for everyone, but most are for small children. This is part of why the reviews include text and discussion: yes it is a good book, but is it good for you?
Finally, there is a smaller category of books that you use and not just read. As I write this, the only examples have been from role-playing games, but others could include manuals, guides, and reference books. These come with the assumption that you would buy them if you plan to use them, since you will need to refer to them. The rating shifts to how broadly useful the book will be, whether I would recommend it for everyone, most, few, or no one, and even then only if you are the sort who would use that sort of book. If you do not repair cars, even the best repair guide will be useless to you; if you do not play Dungeons and Dragons, you are not going to be using those books either.
Or maybe you are like me, own too many books, and read a lot. Feel free to bump everything up a number, buy all the 3s, and help enrich our authors. They are hungry.