If you get on good with your manager you shouldn’t quit in a letter. Ask your boss for a private meeting and then tell them in a very nice way why you need to leave the company. Be sure and let them know how much you like them and appreciate them as a boss. Let your boss know you will write a formal letter just for paperwork reasons and then give them a short, formal notice. If you talk to your boss in person and leave on good terms you should be able to get a good reference out of them.