FTP Access to Edit My Files?

I want to hire someone to make some changes to my web site by editing some of my existing files.

They need FTP access but creating a FTP account for them does not allow them to edit my files.

What is the solution? I would prefer not to give them access to my FTP account, and if it were possible, I would like to limit their access to certain directories.

First, if you’re hiring someone to edit your site, shouldn’t you trust them enough to not have to restrict them to only certain files? You can set up group permissions to handle this:
http://wiki.dreamhost.com/Unix_Groups

Or you can move those directories to another FTP user, then remap them back to your site:
http://wiki.dreamhost.com/KB_/Account_Control_Panel/Domains::_Remap_Sub-Dir

-Scott