Chapter 1 missable achievement - when you are playing as the gnome, and fix the book:
you will leave the library after reading stories, go back in and examine the rotten books in the corner, then talk to the book on the table. he will take you into the past. grab the nails from the desk, then speak to the books on the shelf to find the time travel book in the stack by the desk. he will send you back farther into the past. explore here and find that you need more supplies to continue. talk to the time travel book, but he leaves to learn about time travel. fortunately the stack of not-yet-rotten books can tell you about the past...
look at library, look at workman, take roofing paper, take key, use opening spell with leather cord, take key, talk to old books, go back to the future!
now you can open the toolbox and climb the ladder to repair the roof. look at the books in the corner again and you will return to present day and receive the hat.