The ISTD book was also originally written by Alex Moore. It is more terse than "Ballroom Dancing" so it does not provide as much information at the beginning, but I've heard some hints that at times "Ballroom Dancing" says too much - making the ISTD technique book the best day-to-day reference for the serious dancer.

"Popular Variations" is vastly different from the other two. It is a book of ideas - a collection of phrases a few figures long, including syllabus material, comparable material not on the syllabus, and some very advanced actions. Figures are described only when it is assumed the reader whouldn't know them - and then only briefly. It's assumed the reader already has a thorough understanding of ballroom technique and can deduce most of the details without assistance.

And of course the general assumption is that none of these books are going to serve in isolation - they are to be interpreted through someone's substantial experience - either the reader's, or the reader's teacher's.