Ok, here is an update:
I have taken 2 books which used to be 1 book and put them back together. 392 pages. Bigger font. New cover.
I have gone back to the future and republished several books on my back list with updated covers. I have raised the prices in line with the number of pages. Print cost is less as a result.
I segregated out my books which were part of a series and abandoned the series altogether. I think that people thought that the series was consecutive around the same plotline, which they were not. So if they bought one they felt obligated to buy the rest. This means that possibly all 8 of them will sell.
Next, I reordered my catalog by date of publication instead of genre. Ultimately, I discovered that I had written and published 24 books, 4 of which are now out of print. My catalog is now down to 3 digits instead of 7.
I have been working on a new book (fiction) for 3 years. I am determined to publish it this year. I have started 3 books (nonfiction), 1 of them illustrated, and 2 text only. So I can't say I am bored.
I have gotten over my original funk. 2017 was the worst year ever.
Actually, sometimes a series can be a deal killer. I am discontinuing a series of books and publishing them one at a time, because some readers think they have to buy the whole list instead of one at a time; especially if each book has a different main character to focus on. I am hoping that people will buy one of these titles after the taint of a series has been removed. It will be a distinct improvement from sitting around hoping that someone will buy a single title.
Many of my books run at least 60,000-100,000 words. On reordering the list I have 24 titles, 15 of which are on sale. I can't go any faster, so exhorting an author to have at least 25 books available of such a length is wishful thinking IMHO.