The letter notifies current publishers, including fledgling iBooks Authors, that four areas are being updated: promo codes, screenshots, series and pre-orders.
The iBookstore will now support screenshots, a feature that can be useful for books with illustrations and special audio/visual content.
Publishers are not required to submit screenshots, but if they choose to do so, the submissions must be 1024 x 768 pixels or 768 x 1024 pixels and be in the RGB colorspace. Authors are allowed to remove the menu bar from the screenshot, which would change the resolution to 1004 x 768 pixels or 748 x 1024 pixels.
Beyond picture size, the shots must be in the usual iTunes-accepted .jpeg, .jpg or .png formats and can be submitted via iTunes Producer 2.5 or later.
The new resolutions reflect current iPad hardware, and no mention was made of higher-res images.
Authors can now receive up to 50 free promo codes for each eBook they publish, however the codes are can only be distributed for publicity purposes such as review copies. Outside of book critics, it seems that this change won't affect the majority of iBookstore consumers.
Apple notes that the promo codes will available on to those users with the "Legal role," and are limited to titles that have a status of "On the Store" or "Ready for Store."
Also new to publishers is the ability to put an eBook up for pre-sale before release.
Authors can use the pre-order option without submitting a cover or the book itself as late as two weeks prior to the expected publication date, and the book will show up on the iBookstore once all metadata is delivered.
The letter stipulates that the cover, text and custom preview are required two weeks prior to publication.
Finally, authors are reminded that in order to create a series, their submissions must include series data to more effectively connect all volumes together on the iBookstore.
Full letter below: