I have a habit of falling in love with series that aren't the best sellers in the author's book list. For instance, I love Carolyn Hart's Henrie O series, and I asked her why she doesn't write more of them. Simple answer - they don't sell as well as her other books.
I would love for Simon Brett to write more Mrs. Pargeter books, and I'm eager to read more "art whisperer" books by Charlotte and Aaron Elkins. I can only assume that Brett's Charles Paris mysteries are more in demand, and that Aaron Elkins "Bone Doctor" forensic mysteries are more popular.
What books/series do you wish would continue or come back into print?