..afterthought for my fellow Doctor Who fans--this is precisely what's actually meant by Relative Dimensions In Space. It should be underscored that a book is a 3-dimensional device whose prime purpose is to convey one-dimensional linear feet of text, and in that sense, every book is always bigger on the inside. Try doing this some time:
Measure how long the first line in your book is, then multiply that by the number of lines on a page, then multiply that result by how many pages of linear text is contained by your 3-dimensional book. If you're very ambitious, try that with a dictionary. If you're VERY ambitious, do that with Volume I of an encyclopedia, then multiply that result by how many volumes there are in the set.

That's an enormous amount of linear footage, isn't it.