MacDonald's Happy Meals are just one in a long line of promotions that include something "free" with your purchase. Gas stations have been known to give glasses. A long time ago, there was a Duz detergent that included towels in the packaging. I have inherited some dishes that were premiums for going to the movies. I think these big little books are in the same class.
They are small, only about 3 x 3 but thick. The paper is brittle as you might expect from something about 75 years old. As you can see from the photograph, the covers have retained that brilliance. I am told that some of these are just interesting. I believe that the Tom Swift book is very interesting. Maybe you will recognise some others.