Image size above 1536px for iBookstore? I'm putting together a picture book in iBooks Author.

The images for each page are around 3500px by 3500px.

I'm uploading them as square images, and just leaving a margin on each side of the images.

As I understand, the maximum image size on the short side of an ipad is 1536px.

I can reduce my image sizes to 1536px.

But I'd prefer to keep them bigger so any future ipads with finer resolution, are accounted for. I'd drag the images to the pages in the iBooks Author document and then use the handle-drag things to fit them to the screen size.

But if for users it will increase the download time / make my book run slower, then I'll reduce the images to 1536px.

Or if there's another reason to reduce them, then I will.

Is there reason to make the images 1536px and not bigger?
The main disadvantage is the current iPad will not be able to show the whole image with the high resolution as it was intended to be seen. Unless you were able to make the images a kind of "downloadable content" to view on a computer monitor.  

It would increase the download time of the iBook, but that also depends on how many images are in the book and what the overall finished and compiled file size is going to be.  

I personally have downloaded books that were 1GB directly to my iPad via wifi, but that is because I have fibre optic connection. Also I wanted that book! 

It comes down to what do you think people would want to go through to get your book. Trial and error. There is no "correct" answer for your question. Only advise.