Jerry Uelsmann is one of the legends in darkroom photomontage, you can look at his books for inspiration but you probably already know this.
I personally did darkroom photomontage professionally for over 8 years in a couple of Canadas largest photo labs.
I have to say even back then there were not many good books on the topic if any, you may want to look up articles by an American by the name of Bob Pace, he was not bad but not on the same level as Jerry.
This was a very complicated process with many , many unique ways to work, Jerry for example had over 10 identical enlargers set up in his room so he could move from enlarger to enlarger.
I worked with the Lisle Camera which allowed for complex masking techniques and butting of images together- this camera and operators was very limited in NAmerica and so few have heard of it.
It would take me 9 solid months to train a technician to be proficient in most of the skills , when I trained it was the worst 9 months of my professional career, but one day I walked into work and it all made sense.
We worked on many complicated overlap images sometimes combining over 50 images on a single sheet of 16 x 20 inch transparency film.
I have not seen many good books on the subject as there are no photographers that I. am aware of with the skill set people like Jerry around today or even back in the 80's when this was a popular method of producing work.