Translation:Dit boek is het kleinste van alle boeken.
Why is kleinste used instead of kleinst? I thought it ended in -e when it was used as an adjective, like in het kleinste boek...
You are quite right! It is the adjective form. If you want to drop or imply the noun (for example to avoid repetition or because everyone already knows what you are talking about), you can use just the article + the adjective that would normally come before the noun as a stand-in for the noun itself. So here "het kleinste" means "the smallest one" (and the "boek" is implied). Another example: "Wil je de grote of de kleine?" means "Do you want the big one or the small one?"