What to write in a chapter of a book? Recently, I got an invitation from a professor to contribute a chapter in one of his/her upcoming books. I am a novice in this field. I am still a student but simultaneously I do not want to lose this opportunity. I have a few published articles. I am quite confused about the content of the chapter. Can I use my results from the accepted manuscript and add them as chapter content of the book? Is it totally ethical? Kindly help me. Suggestions are most welcome by heart. Thanks a lot. 

P.S. Checked their profile. They are some professors in an university but I never met them. And they did not talk about whether its a normal chapter or some proceeding type chapter in the book. And no mention of money also.
Rather than answering the specific question ("What to write in a chapter of a book?") I'm going to suggest that this is an XY Problem. The real first question is whether this is a real book, or if it's a predatory publisher sending out an email blast to random victims. The limited evidence in the question and comments suggests that it may well be a predatory publisher:


The recipient is not a well known researcher 
There isn't a clear and well-described topic for the chapter
The recipient doesn't know the putative professors personally


An edit to the question suggests that the authors may be real people, but with the other red flags that are raised I would still be very suspicious. The professors may not be the actual senders of the letter, or they may in turn be victims of a scam. 

It's not impossible that this is a real book, but the odds are very much against it. Please be cautious before doing something that may damage your reputation and career.

Edit since in the comments someone is skeptical that there is such a thing as predatory book-chapter scams:


Example 1
Example 2
Example 3
Example 4


The claim in the comments is that book chapters can't be predatory because "You can't have a predatory book". These examples show charges for chapters, but also through selling worthless, low-quality books to libraries. 

In general these predatory publishers depend on naive people; that includes people who think "You can't have a predatory book".