Help! Fetch xml from an external source, then pass it into a form.

OK, I'm looking for tips and help regarding fetching xml data from an
external source and
then parsing it into a form.

What do I need to make this happen?

Example:
I have an Advert Model (which is for books)

fields do
   isbn :string
   title :string, :required
   author :string, :required
   edition :string
   condition enum_string(:As new, :Good, :OK, :Bad)
   price :string
   timestamps
end

I want to allow the user to click on the "New Advert" link in the
Adverts index page, enter the isbn number, then press a button or
click a link to do a ajaxified lookup to an API service like
isbndb.com to get the rest of the data about the book like author,
publisher and populate those fields on the New Advert page, and allow
the user to review the data before clicking the Save Advert or Cancel
button.

Would love some ideas or help. Anything would be very much appreciated.