The resolution will be whatever the iPad is creating, so a Retina iPad will have 4 times the number of pixels as a non-retina iPad.
You can probably find the correct spec for each model on Apple.com, a quick search found this…
http://www.theipadguide.com/faq/what-resolution-ipad-retina-display
It suggests a Retina iPad is 264 pixels per inch, at 2048 x 1536, or 3.1 megapixels.
A non-retina iPad is 1024 x 768, I think the iPad Mini is another set of resolutions too.
Take your screenshots with a Retina iPad & it should be fine, upscale the images if you really want 300dpi. You will probably need to convert to CMYK if this is a print book so add both tasks as an action in Photoshop or whatever image editor you are using. Then you should be able to apply that as a batch to take out the mind numbing process of manually doing each one.
Automator may also do a 'batch upscale', but tools like Photoshop support extra plugins that can provide better upscaling like…
http://www.ononesoftware.com/products/resize8/
If you use Perfect Resize 8 it may be OK to upscale even non-retina screenshots.
Jay-Ray, screenshots via Reflector on a Mac look worse than ones taken on the device, are you sure about the quality that Airplay streams? Perhaps it is my non-retina LCD causing it.