If referencing a singular type of book, the first sentence is correct as is. If referencing multiple types then the sentence would be "These kinds of books are sold by all booksellers." The second sentence is also singular since it is 1 set of something. The word "keys" just describes what the "set" is comprised of. If there was more than 1 set then it would read as "The sets of keys are on the table." or " The key sets are on the table.". Though I believe most people would shortened all of the sentences, both the singular and plural forms, to just "The keys are on the table".

>Now I think that it is a logical problem or it's just a fixed expression without any reasons,so it cannot be explained grammatically. I see your what you're saying. The words "kind" and "set" are a bit different, though. A "set" is a group of things. If you have a set of keys, you have several keys. You can't have just one key and call it a "set." A set has to be more than one thing. A "kind" is not really a group. A "kind" refers to the properties that something has. It's a way that something can be. For example, a novel is a kind of book. A novel is not a group of books. It's a description for books that have certain features. If a book has these features, then we call it a novel. We can say that a certain author has written "a new kind of book." This means that the new book is not like any other books. There is only ONE book of this kind. The book doesn't belong to a set or group. There are no other books of this kind. There is only one. But we can still talk about what "kind" of book it is, even if there is only one. By contrast, we can't really talk about a "set" of books if there is only one book. It's not a set if there's only one. It's a bit hard to explain, and I can see why it might be confusing, because we often put things into a group together after we realize that they are all the "same kind" of thing. If you find it easier to think of a "kind of thing" as a set expression, that's probably fine!

“This” is singular so it would be book. If you wanted to refer to multiple kinds of books you would say those.

Now I think that it is a logical problem or it's just a fixed expression without any reasons,so it cannot be explained grammatically.🙄