As a professional historian who routinely reviews and proofs history books for publishing houses all the time as part of my job, I can tell you that a year and a half, believe it or not, would be a lightning fast turn around time. Most books go through DOZENS of drafts and have DOZENS of editors commenting on them. Each time an edit is made, it must be sent back to the author for their approval, and then it goes back, and on and on. Decisions about paper, artwork, typesetting, the cover, and so on all have to be made. It took me almost 4 months just to get the copyright back from my dissertation alone (government red tape, of course)! Photos might have to be cleared. Maps need to be created. All of this is done on other people’s timetables, not yours. It is a shockingly complicated process.