Learn Spanish (Spain): Leo ocho libros en un año.

No, the numbers 2 through 10 never change their endings. You will always use ocho whether you are counting masculine words like libros (books) or feminine words like casas (houses).

Does un mean "a" or "one" in this sentence?

It can mean both! In Spanish, the masculine word for "a" (un) is exactly the same as the number "one". Saying "I read eight books in a year" or "in one year" means the exact same thing here. Just remember that unlike ocho, the number one does change its ending to match gender (becoming una for feminine words).

Where is the word for "I" in this sentence?

The "I" is built right into the verb. Leo is the "I" form of the verb leer (to read). Because the -o ending already tells us that "I" am doing the reading, adding the word yo (I) isn't necessary in Spanish.

Why is there a wavy line over the 'n' in año?

That wavy line is called a tilde, and it turns the letter into an ñ (eñe). It produces an "ny" sound, similar to the middle of the English word "canyon". It is a completely separate letter from the regular n. It's very important to include it here, because año means year, but ano (without the tilde) means anus!