What would be the best dimensions to design an book, for viewing (vertically) on the iPad. I know that iPads have changed dimensions through the years, so what would be the best dimensions in order to cover viewing from the first iPad to the latest.
All iPad screens, from the very first version to the latest ones, have a 3/4 aspect ratio.
Therefore, scaling does not affect content size proportions. A circle remains a circle when scaled.
As for resolution: a good base resolution would be 1536x2048. (newer models do sport higher resolutions, but in terms of bitmap graphics file sizes and sharpness still a good tradeoff)
What kind of book are you considering to create? Which output format - epub? Fixed layout or reflowable?