This is the book about which I told you. (into a compound sentence).

b.Besides being rich, he is learned. (Compound) 

= Not only he is rich but also he is learned.

In order to stay healthy, you need to exercise regularly. = You need to exercise regularly and stay healthy. 

A compound sentence is a sentence that connects two or more independent clauses, usually with a coordinating conjunction (for, and, nor, but, or, yet, so) or a semicolon. Each independent clause has its own subject and verb and can stand alone as a complete sentence.

Example: "The sun was setting, and the birds were chirping melodiously."

A compound sentence combines two or more independent clauses. An independent clause is a group of words that contains a subject and a verb and expresses a complete thought. These clauses are typically joined by:

• Coordinating Conjunctions: These are 'for, and, nor, but, or, yet, so' (FANBOYS). When a coordinating conjunction connects two independent clauses, it is usually preceded by a comma. For instance: "I went to the store, but I forgot to buy milk."
• Semicolons: A semicolon (;) can be used to join two closely related independent clauses without a coordinating conjunction. This implies a closer relationship between the ideas than a period would. For example: "She is a talented musician; she practices daily."
• Conjunctive Adverbs: These adverbs (e.g., however, therefore, moreover, consequently, nevertheless) can also join independent clauses, but they are usually preceded by a semicolon and followed by a comma. For instance: "The project was challenging; nevertheless, we completed it on time."

Compound sentences help to show a relationship between two equally important ideas and add variety to writing. They are distinct from simple sentences (one independent clause) and complex sentences (one independent clause and one or more dependent clauses).