20 years in the making

His book was 20 years in the making. (in the making = in the process of being made or produced)

Does the above sentence mean that he spent 20 years writing his book?

 

I think the sentence says it took 20 years for the book to be what/how it is now. The sentence does not necessarily imply that he spent 20 years writing his book -- only implies that the book took 20 years in making, I think.