Cool puzzle book overall! I really like how the the idea of themed word searching works well with word choices and the puzzle designs, especially the visual designs in 8, 9 and 10 and the small jokes in 3 and 6. The letter arrangement in individual puzzles also makes them more solveable, beautiful and interesting, even in pre-0 levels.
The only two puzzles I found less appealing than others are 1 and 11. I personally knew nothing about cookies so the four given words in 1 on the left made me really confused. Even after knowing the theme, I could only do the puzzle by ctrl+F-ing the first letters of hidden words on Wikipedia, which makes it less fun than all other puzzles where I can search for familiar words in the pdf rather than searching for seemingly random words online. In 11, the theme is clear, but although knowing the theme makes searching for most hidden words easy, it helped little in finding "category" and "leftovers", since the themes of 11 and 12 do not directly lead to those two words. The only source of those two words seem to be the wordsearch itself, which is made difficult by the significantly large number of leftover letters. (Also, the titles of 2 and 11 were not helpful because I believed in the word choices "animal" and "theme", which prevented me from reading the titles.)
Other levels are fun! In addition to the jokes and visuals, I like most of the titles, the design of fake dots in 5, the entirety of 6, the good demonstration of the theme in 12 through using the answer of itself as a word searched for.