When requesting a leave of absence that spans 2 whole years, the best thing is to write a letter that includes the following points:
- The exact dates you will be away for
- Some indication as to why it is imperative that you take this leave.
- A reminder of how committed you are to your employer, and why you would like to return after this period of leave.
- A request to discuss this further in a face-to-face meeting.
Although you asked for a letter explaining your request for leave, I am of the opinion that, in normal circumstances, NO employer would allow an employee to go on leave for two years and then agree to hold the position open for his return.
If you have extraordinary circumstances, then the appropriate way to relay that information would be to talk to your boss directly, discuss your plans, and ensure that he has all the information he needs to make his decision.
To simply write him a brief letter requesting two years of leave and expecting him to grant it would be considered unprofessional at very least, and impolite possibly.