Apple has made a number of changes to the iBookstore so publishers can now use screenshots, promotional codes and offer pre-orders.

Apple outlined the changes in a letter to participating publishers this week. The iBookstore is open to publishers and individuals who want to submit their own eBooks to iTunes and the changes are designed to help them make their works more appealing to potential buyers.

Firstly, the iBookstore now supports screenshots, though these must be submitted as 1024 x 768 (or 768 x 1024) RGB .jpeg, .jpg or .png format files. These should be submitted through iTunes Producer version 2.5 and up.

Publishers can now also receive up to 50 free promo codes for each eBook they publish, but these must be distributed "for publicity purposes" only, Apple insists.

If authors or publishers want to link a series of eBooks, they can now do so, provided they give the correct information when delivering their content - the name of the series and the number in the series, specifically.

You can now also set up pre-orders. "You can make your book available for pre-order without submitting a cover or book asset up until two weeks prior to publication date. Your pre-order will appear live in the iBookstore as soon as you deliver complete metadata," Apple writes in the letter.