If you are working in Fixed Layout Books, the resolution of the page is what you define it to be, however 738x985 is what Apple says their full screen, single page resolution is in iBooks.
Individual images can have up to 2million pixels (LxW). Having a bigger image, that you scale down to the max resolution is always better, if the file size of the image doesn't affect performance.
1107x1478 is 1.5x the size of the single page resolution, and is 1.6 million pixels. Doing a bit of jpeg compression should yeild manageable file sizes, and look very good. I try to keep my backgrounds 400kb or less if I can.