@emma_woodorth, I never addressed your question about the music books. If you regularly stock a lot of them, creating them as inventory will help you track your stock and allow you to invoice for them at a markup, etc. But I wonder if that would really be necessary.
If you obtain a book for a known student and invoice them for the books immediately, you could use Disbursements. That tab was designed for pass-through expenses, although it also allows you to mark up or discount the expense.
Another possibility is to allocate your music purchase costs to an expense account (you could even name it Students’ Music, for example). When you sell the book to the student, you would Receive Money and allocate the transaction to that same account. Manager will automatically enter the amount as a negative because you are receiving money in what is normally an expense account (where the normal transaction would be to spend money). This will work conveniently if you do this only occasionally.
Still another way is to allocate the purchase costs to Students’ Music, but receive payments to an income account (which could be named something like Students’ Music Payments).
The final choice really depends on how many such transactions you will be making and what information you want to extract from your records.