Deleting in update - lazy and bad form or ?

I’m a bit confused as to how I came to this solution but thought it might need some review.
I have a number of fields that can be edited by the user. As an example, let’s say I have 5 text fields where
a user can list up to 5 of their favorite Rails books, in order of favorite.

1- Agile Web Development
2- Ruby and Rails: Up and Running
3- Ruby for Rails
4- Build Your Own Ruby on Rails Web Application
5- Ruby on Rails for Dummies

Tomorrow they edit the form, move 4 and 5 off, so they are down to three books
A few weeks later, The put Ruby and Rails Up and Running as number 1 and Agile Web Development as number 3

So you get the point. Well, while I sort of know I can do some comparing , size, value checking and so forth, it seems easier
to just delete the current entries on update and treat the changed selections as new.

I wanted to get some opinions and see if this is an okay thing to do ?

TIA
Stuart