how to register publishing company?

My friend is a part time writer. Now his book sales are very good and starts to make profit, such as from amazon, BN, etc. What type of company should he register? He aslo have regular job as a full time engineer.He has costs to self publish books,such as to pay book cover, editor, he uses home to write his books, etc.

He is married. The couple's income are very good as engineers.

which type of companys should they register? should it under both hu*****and and wife's names?