Learn Spanish (Spain): El libro no es fácil.

In Spanish, to make a sentence negative, you simply put no right before the verb. So, instead of saying "is not" like in English, Spanish says "not is" (no es).

Would fácil change if we were talking about a feminine noun, like a lesson (la lección)?

No, it stays exactly the same! Adjectives that end in a consonant (like the l in fácil) or the letter e generally have the same form for both masculine and feminine nouns. You would say la lección no es fácil.

Why do we use es instead of está for "is"?

We use es (from the verb ser) because being "easy" is seen as a fundamental characteristic or quality of the book. The other verb for "to be", estar, is used for temporary states, feelings, or locations.

Why does fácil have an accent mark on the first letter?

The accent mark tells you which syllable gets the extra push when you speak. Words ending in a consonant other than n or s naturally stress the last syllable. The accent mark breaks this rule so you know to pronounce it FÁ-cil instead of fa-CIL.