An average book page has about 250 words. If you have a good subject you should be able to knock that out in 30 minutes. After 365 days, you have a 365 page book. Of course, then there is the editing.....
I remember doing a paper for a supervisor who wanted 500 words on a subject. I started writing and didn’t realize I had written about 600 words before even getting a good head of steam up.
I really need to get a new job.