I advertise sales of sports memorabelia items from my website sites. I use web links, banners and graphics and have not had any probems in 12 books so far.
How you finance your production is not a concern for Apple, remember not all people use iBA to make books, some pay others to do them and deliver to the iBookstore.
Better to keep any "adverts" to the end pages of the book rather than infiltrate them into the main body..is my opinion. Or you could use a Sponsors page to acknowledge them.