I recently read an interview with John Grisham in the London Guardian. I'm not sure I'd include Grisham in my list of "mystery writers" - I think most would agree that he falls into the category of "thriller" writer. For me, the blockbuster mystery writers are Laura Lippman, Harlan Coben, Janet Evanovich, Patricia Cornwell, P.D. James, Ruth Rendell ....
In the article, Grisham discusses one of his secrets of success, a tip shared with him early in his career. An editor said to him, "The big guys come out every year." By that, he meant that Stephen King, Michael Crichton, Tom Clancy, et al. published a book a year, thus keeping themselves in the public eye. And, looking at the mystery luminaries in the previous paragraph, I think we can see that many of them do at least one book a year, and sometimes more than that.
I'm not sure Grisham is giving himself enough credit, though. Clearly, he's hit on a formula that people like. I don't read much Grisham; I want as little to do with lawyers as possible. But I've read a couple, and they've been fast and entertaining, with good twists and turns.
And, on the other side of the coin, there are writers whose fans salivate for years waiting for the next book. The late great Sarah Caudwell was one of them; and I wish George Dawes Green would write more. In a completely separate genre, Jean Auel's fans followed her over the course of (I think) 7 books and 30 years.
A book per year is a lot to ask of a writer; but I can think of many writers who'd love nothing more than being able to do just that. And yet, in and of itself, a book a year isn't enough to get you to the top. I can think of two dozen writers off the top of my head who've published at least one book a year (the late Dell Shannon/Elizabeth Linington used to do three) who never get out of the 5-10K range in terms of sales per book. That's respectable and can pay some of the bills, though perhaps not both the mortgage AND the car payment.
The publicist/marketer in me understands the need to stay in the public eye. There's something annoyingly fickle about the American public. It's a machine that has to be fed. With so many people (read: musicians, novelists, journalists, actors) vying for attention and those precious few top slots, "if you snooze, you lose." Look at Lady Gaga. Say/think what you will about her (I'm mostly a fan, even if she occasionally becomes tiresome), she knows how to keep the public interested. That's no mean feat. If we think of books as commercial products - and I do - then it makes sense to stay in the public eye, to keep building your brand. Michael Crichton knew this; so does Stephen King. Both, I think, also had the vocations to be professional, full-time writers, which doesn't hurt.
The other side of the coin, of course, is the question: How good can a book be when a writer whips it out in three months? With some writers, it doesn't matter. For instance, I like or love almost everything Joyce Carol Oates writes, and they don't come more prolific than that. But others (who shall remain nameless)... oh Lord, have the recent books been total garbage. As an editor, I say: Get them out the door, bring in the revenue, pay the bills, reward the staff. As a reader, I think: If this manuscript came to me as a first submission by an unpublished writer, I'd get out my rubber REJECT stamp by page 10. It is another curious American phenomenon that word of mouth about a book can be so bad and yet people keep buying more books by the same writer. (Think, for example, of a book that 1,500 reviews on Amazon, of which 1,000 or more have 1-star ratings.) Reading those reviews, I think hope springs eternal; devoted fans are hoping for a return to form for a favorite writer (this rarely materializes). Also, to give these writers their due, they have succeeded in crafting a basis and a character that keeps readers wanting to know WHAT HAPPENS NEXT. That is no small feat.
I have an author on my list - one of my favorites of my career. S/he is always brimming with ideas. The manuscripts come to me pristine. I feel like less of an editor because s/he just "gets it" and needs so little input from me. I say, "Give me more books." S/he replies, "Agatho, give me more money so that I don't have to work for a living, and you'll get more books." Alas - the constant dilemma. Because author X might be able to give me two books a year under two different names, but that doesn't guarantee that we'll be able to sell scads of them. But we try, oh how we try!