"He has twice as many books as I do."
Translation:Él tiene el doble de libros que yo.
Because their term for double is "the double." It's kind of like how we say "double the amount of..." instead of "double amount of."
él tiene doble tantos libros como yo --> is this not correct, because you use tanto or tan como only when it is the same amount/the same?
I put "Él tiene el doble de los libros que yo". Putting los in there made it incorrect but I still think it's gramatically correct.