The puzzle type that he called Pawn Mower has existed before in various names; one that I know of is Goishi Hiroi, created by nikoli. (The puzzle type has since been phased out, but googling for that name gives various mobile apps for the puzzle type.)
When you're only using the chess pieces and their movements, without using more things like having two players taking turn etc, I wouldn't exactly call those chess puzzles. On the other hand, if you're looking for that kind of puzzles, the chess section of Erich's Puzzle Palace can be a good start.
Finding good ideas is hard; if it's easy, then we're all artists, writers, and musicians, among other art forms I can't think of.
I am writing a chess puzzle book for my high school senior project! I want something unique and fun. You guys for sure know who Maurice Ashley and how he invented pawn mower. This type of chess puzzle is so much fun and very unique. How did he develop this idea? And how can I develop an idea as unique as his to write my chess puzzle book.