I have been doing research on my latest book for at least five years. Occasionally I start a book and decide I already know what this author is trying to say. I tend to set those books aside. Now I am trying to finish reading all the ones which seem relevant to my latest book. This is mostly boring. However, every once in a while I find something unexpected. So, although largely bored, I persist.
But, I think I am reaching the point of diminishing returns. Even so, I want to finish reading the two paper books I have left uncompleted. Then I think I will finish off three more ebooks. After that, I think I will accept I have done as a valid research effort and write the conclusion to this latest book.