One of his novels is interesting.
There is only one of his novels that is interesting.
Explanation: "that is interesting" is a restrictive relative clause, and it relates to the noun phrase "one of his novels". Basically, it relates to the word one.
We can use are here if we say that:
- There is only one of his novels (in this room, or on the table).
- By the way, his novels are interesting.
We would have to use a comma in this case:
There is only one of his novels, which are interesting. (his novels are interesting)
We can also write the sentence this way:
There is only one of his novels, which is interesting.
- There is only one of his novels (in this room, on the table, or elsewhere).
- This fact (that there is only one, and not more) is interesting.