C#: Keeping an xml file in sync with data model

[l2player]

Lets say I have a \[code\]List<Book>\[/code\] that a user can add/remove books to/from. The list is persisted to an xml file so that the data is available between sessions on a website.Now, obviously this is not an ideal solution, a database would be much nicer. But this is for a school project with some rigid constraints. What I'm trying to figure out is if there is a good way to "update" the xml file whenever the \[code\]List<Book>\[/code\] changes instead of simply rewriting the entire file each time.