Learn Spanish (Spain): El libro de los animales es pequeño.

In Spanish, to make a noun plural, you add -s if the word ends in a vowel (like libro becoming libros). But if the word ends in a consonant, like the l at the end of animal, you must add -es to make it plural.

Why do we use los before animales?

Los is the plural form of el (the) for masculine nouns. Because animal is a masculine word (el animal), its plural form "the animals" becomes los animales.

Since the sentence mentions multiple animals, why isn't pequeño plural (pequeños)?

The adjective pequeño (small) is describing the book (el libro), not the animals. Because there is only one book, and libro is a masculine noun, the adjective stays in the singular masculine form pequeño.

Does de los animales mean "of the animals" or "about animals"?

It actually means both! While de literally translates to "of" or "from", Spanish uses it frequently to say what a book, story, or movie is about. So el libro de los animales is the natural way to say "the animal book" or "the book about the animals".