"I read a lot of books in the summer."
Translation:Yo leo muchos libros en el verano.
I also used 'leo' but it occurs to me that 'leí' should work as well. Did anyone try it?
"Leo muchos libros en verano" is correct. You can say "en verano", "en otoño"... without "el", it is totally right in Spanish.
Why muchos and not mucho like the example below?
'They write a lot on the weekends." - Ellas escriben mucho los fines de semana.
In the sentence above, "muchos" is an adjective, modifying the noun "libros". Since it's being used as an adjective, it has to match its noun in number and gender.
In your example, "mucho" is being used as an adverb, to modify the verb "escriben". Unlike adjectives, adverbs don't change to match number or gender.