Learn Spanish (Latin America): Usted lee un libro corto.

In Spanish, descriptive words (adjectives) like corto usually come after the thing they describe. So, instead of saying "a short book," you say the equivalent of "a book short" (un libro corto).

Why does corto end in -o here?

Because it is describing libro (book), which is a masculine word. In Spanish, adjectives have to match the gender and number of the noun they describe. Since libro is masculine and singular (just one book), the adjective takes the masculine singular ending -o. If you were describing a feminine word like carta (letter), it would change to match: una carta corta.

Doesn't lee mean "he reads" or "she reads"? Why is it used with "you" here?

You are right that lee is used for "he" (él) and "she" (ella). However, usted is the formal way to say "you," and it always shares the exact same verb conjugations as "he" and "she." So, usted lee means "you read" (formal).

Can I use corto to say a person is short?