Royalty split.. How can I see what's sold?

Depends on who "owns" the paid books account!

Sales information is found using the accounts user and pw for iTunesConnect > Sales and Trends.

Or if its not going to be you...the other party could supply a copy of the sales statement.

Unless the author has some standing or the book is exceptional....sales can be few and far between unless some serious advertising/ promotion is undertaken.

You could ask for an up front payment..... to cover your time....and share any sales.

If the book sinks, the author loses nothing.... you lose commission.

Forgot to mention.....Apple take 30% off the top.... the balance goes to the author....and may be taxable when added to other/ main income on annual tax returns. The author would then claim whatever you were paid to offset his/ her totals.