In the jazz area, in the "1200 songs" sticky thread, you can download the songs that make up a pretty comprehensive list of songs from several "real books".
It might be a bit lengthy, but you could download that list. What happens is:
- tunes you already have are not duplicated
- tunes you modified are
What you WILL have, though, is a playlist with those 1200 songs, so you could match them up with the real book you're interested in and delete the unwanted ones (which will stay in your basic library if you delete the playlist entry, not the song, of course).
The unmatched, different songs in the resulting playlist will have a "1" added to them (or other number, in order, if you already have duplicates).
Hope that helps, and is clear.