Can there be "too many books"

about a subject? Yesterday we had friends unexpectedly come for the evening meal. Like my father they were teachers and, like my father, the husband was the headmaster of several particularly interesting schools.
When my father ceased headmastering he turned to other pursuits, conjuring, making conjuring apparatus, other woodwork, gardening, and reading. He still does these things.
His friend took up residence on a 20acre block and became involved in the local rep theatre and reading. He also wrote a book about his experiences as a headmaster. He sent it to a "contact" in a reputable publishing company who told him there were "too many books" using that subject matter.
Now I wonder if that really was the case or whether the contact thought that the book was simply not good enough? Is it really possible to have "too many" books about a subject? It seems to me that if what is said is being very well said in a new and interesting way then there has, potentially, to be a market for it.
It also happens that there are new, badly written books out there about subjects which have had a great deal of exposure. When knitting became very popular again there were a number of badly written books that appeared very quickly, far too quickly. There were books which followed on the success of Harry Potter. Some were good and some were not so good. Some were terrible.
So, the reminiscences of a headmaster? I suspect there was room and there still is room. It may be that someone else would have been interested. There is also the possibility that it simply was not good enough.
But, can you have too many good books?