I'm not sure about the differences between 'nerd', 'dork', and 'geek‘, but I think 书呆子 and 宅男 are different in Chinese. 书呆子 is an old saying and often used to describe someone who is smart (but not extremely smart) but too addict to reading books and believing what he reads. Mostly, when we call someone 书呆子, we don't mean to critisize or judge him. It's just an figurative or ironic way to show opinions and care. 宅男, which is what "geek" means, is a new word in Chinese and originates from Japanese. You know 宅 means HOUSE, so 宅男 means guys who spend most of their time at home instead of going outside for exersices, meals or tours. However, now 宅男 is more like a cultural sign and stands for a certain group of people who shares the same interests in cartoon, entertainment stars and so on. You don't have to actually stay at home all the days to be a 宅男, but being a 宅男 requires some certain characters.

This is the way I see it, 书呆子 = people who do well academically but bad at everything else 宅男 = A negative Japanese term, referring to a shut-in or someone with no social life.