Hi Matthew,
You could potentially use the time and item lines to vary them - item lines for books, and time event line for courses. You can add the selected type of line here:
The difference is mainly the headings - ones time based, the other is quantity based:
The headings are customisable under Account Settings > Invoice Customisation.
Select the items box in the middle, and change them (although bear in mind this changes it for all invoices, past and present):
As for the number of delegates, the only thing I can suggest is adding the number to the description/notes field, or add it as a separate comment line.
I hope that helps?