You are going to get several and varied opinions, Richard. However, if you are changing ages, like you did in your recent sales thread, then I don't see a problem with it. I believe october has had separate threads for both GA and SA books. Bob Siman has created different threads for each batch of books he has sold over the last week or two. I think he is on thread number three now.
So, I really don't see it being a problem. Now, if you only listed one book per thread, unless they were all keys and worthy of such treatment, that would be a different story altogether and frowned upon by the forum police.