Yes, Bill, you're right. Maybe looking specifically for a guitar book isn't the right way to go about this (guitar is such an awful way to understand theory, anyway, because of the six strings). One has a disadvantage not knowing the keyboard when undertaking the study of music theory.
Piston's early chapters in "Harmony," on Harmonic Progression, Scales, Tonality, Modality, Harmony, Figured Bass (well, maybe not Figured Bass), are as relevant to today's pop music writers as they were to the music majors of the 1940's.
Piston was an academic and wrote for young aspiring academicians. Fortunately, today's publishers have catalogues full of music theory and music instruction books on theory by the cart load -- you can generally find a harmony instruction style book in many of today's relevant idioms if you look hard enough. Try shops like Aebersold.com, who carries much more than jazz play-along these days if you're interested. Even Berklee has branched beyond jazz to cover other genres and techniques.
And no, there won't be one, single book -- learning is a lifelong habit. I am preparing to move, and I'm paying the price of my habits -- packing and
schleping my book collection until I get resettled, whatever that looks like. But get in the habit of reviewing and buying books that will be useful for a lifetime. That means spending some time with a book before you purchase, and it's a damn shame we don't have the book store, "brick and mortar" like we once did, were a person like myself could wile away the hours choosing between Mehegan or Andy LaVerne might be a bit more appropriate for my particular improvisational needs that day. (It's also nice to live in a town like San Francisco because of neighborhoods like Clement St. with its fascinating used book stores (I bought a fair copy of Piston's Counterpoint at Green Apple Book), and dusty cafes designed for quiet contemplation or thoughtful discourse with a total stranger, or strange character -- either.