Of the top of my head, I suggest you watch 'Alien', 'Halloween', 'Psycho' and 'Jaws'. You should be able to find plenty of books about these scores too, because they're all classic films.
Newer horror films usually employ pretty similar techniques but I think one that stands out of the crowd is 'Silent Hill', which has an incredibly beautiful and evocative soundtrack. You should pick something typical as well, like 'Saw' or 'The Descent' or something, to show the kind of methods that are usually used in contemporary horror.
I think you should consider mentioning a horror film with no music as well, like 'Blair Witch Project' or 'Rec', because they have very good reasons for not using a soundtrack, and at times something can be more powerful without music than with it.