This book is worth reading. "Worth" is an adjective here. Adjectives generally cannot have -ing clauses as complement. But "worth" is an exception. Consider also: It's an idea worth considering. His story is worth checking out. Note that "worth" can also be a noun in examples like A dollar's worth of chocolates and You should make an estimate of your net worth. But "worth" is never a preposition. PaulM
Thank you, Paul!