Many academic authors come to me caught in this interesting spin-cycle of research and note-taking. This period can last for many months or even years, and some authors never leave it. I know of at least one project that was in the research phase for over twenty years! The resulting lovely book went on to win a Pulitzer, so the effort wasn't wasted, but those of us who know the story well also understand that it didn't have to take that long. And yes, like the woman above, the author probably forgot much of the early research and had to re-trace his steps a few times.
Some authors strike a productive balance between research and writing. Many focus on one aspect of the book at a time (this requires discipline, because it means not reading everything else that comes to hand, and not checking out every possible book on the larger topic, but noting them on lists for later). They then they write up the research on just that bit, even if they know it will change later.
Meanwhile, some of those who get stuck in the research phase also become overwhelmed by potential sources, and a few express fear at the notion of having to change anything after writing it. They don't want to step out of the research phase long enough to write because they dread learning something new that will overturn their conclusions. But why? They're just words on paper. You can change them. Promise yourself that you will change them if you learn something new, but if you're stuck in the spin cycle, try writing your research up in tiny-bit text form anyway. Not only will you make steady progress, but you'll have the luxury of drafting your book while you're still fresh on each source. This is also a great plagiarism prevention device, because it's easier to check and make sure you didn't inadvertently copy anything without attribution.
Writing bits of actual text as you go and paying meticulous attention to the notes will yield something amazing -- a book that grows bit by bit, organically, rather than being rushed along in a too-fast writing phase. This can potentially yield text that has more depth, richness, and character.
Image of Ukraine’s Central State Historical Archives in L’viv borrowed from www.usukraine.org.