A novel in the form of letters is called -

♦ Epistolary novel:
- A novel in the form of letters.
- The narrative of this type of novel is carried forward by letters written by one or more of the characters of that novel.
- Richardson's Pamela is an example.
- অনেকটা চিঠির মতো করে লেখা এই উপন্যাসগুলো।

Option,
• Picaresque Novel - A novel that tells the story of a rascal or deceiver who chases adventure and duels with the evil villain. It is realistic in manner and satiric in aim.

• Unusual Novel: There is no such category of Novel in English Literature.
• Regional novel: A novel that depicts in its plot the customs, dialects, and ways of life of a specific ritual.

Source: An ABC of English Literature by Dr M Mofizar Rahman and Britannica.