When I finish my history degree I will have to search for a job. This is fine. I love to read historian's books. Sounds pretty odd to make it a possessive like that. You could say 'I like to read books by famous historians' or something like that. But if you simply want to say you like reading books about history then you say 'I like reading history books.' Or 'I like to read books about history.' I have no idea if this is used in other English-speaking countries but in the UK we say 'to do a big shop' to refer to going to the supermarket, normally once a week or so, to buy lots of stuff at once. 'I prefer to buy twice a month' is correct, although it's a lot more vague than talking about doing a big shop.

When I have a degree in History, I will have to look for a job. Instead, I'd rather go shopping twice a month.

Finish my degree, to do a big shopping are correct. But the next few sentences need some tweaking. I am writing an essay on education (?). Could I write: When I finish my degree in history, I will have to search for a job. I love to read books on history ( or books written by historians) ?? Also hobbies (empty statement) I prefer to do a big shopping instead of shopping twice a month. Thank you.