simulacrum Not dropping skill book

After beating the 15th wave I get all drops besides the skill book and the level doesn’t go up. What’s going on.

Make sure that you've allocated all of your delirium atlas skill points. For example, if you've defeated the boss once already and gotten/used the first book, then you will have 2 points to use. If you don't use those 2 points (where at least one of them is "+1 to simulacrum difficulty"), then the lvl 2 book won't drop.