From Beaver's perspective, it seems like his father spends a lot of money paying bills. Ward makes a statement that all adults worry about money unless they're millionaires, which he will never become working at his job. So when Beaver heads down to the bookstore to buy a book for himself, he instead buys his father a book he sees in the store window, that book titled "How I Became a Millionaire in 12 Months". Beaver believes this book will solve all Ward's money worries. After Ward makes a fuss about the book and mentions that he will both read it cover to cover and put it in a place of honor in his library, Beaver is certain that their family will become rich. So a disappointed Beaver doesn't understand why he finds the book in the kitchen's cookie drawer (where June hastily put it when she needed to answer the phone).
—Huggo