When I got my instrument rating eons ago, Kings were still using VHS tapes and their prices were not much different from today's prices--which of course meant because that was in 1975 dollars, they were really pricey--in today's money, that $575 would be $2,500. So although I'm a fan of Kings, because in spite of their hokey humor and Martha's hairdo, they do a marvelous job of getting the information across, I opted instead to use the FAA's Instrument Flying Handbook, suppemented by Taylor's Instrument Flying and Buck's Weather Flying. The FAA's IFH has been updated a lot, although it's still pretty dry--but it's all there. Both Taylor and Buck give more "real world" explanations for why we do what we do. Taylor's book has been regularly updated; Buck's has not, but it's still well worth getting.
Back then, the IR written was allotted 6 hours to take; I took less than that, but not a lot less, and did well on it. As with all FAA written tests, it had more ancient stuff than necessary, as they seem to be at least 20 or more years behind. I had to answer questions on 4-score AN approaches, how to find the "null" of manually tuned DFs, how to avoid being 180 degrees off when tuning manual DFs, the correct order of reciting reporting points in a non-radar environment, etc. (That last one I've actually used, even as late as 2 years ago.) BTW, I aced the AN approaches because some still existed in Alaska when I was a private student up there, and for the fun of it, my instructor had showed me how they were flown.
Get to studying, whatever method you use, because it will make a lot more sense in the cockpit. When you do get into the cockpit, if your CFII is any good, he/she will stick with the basics, mostly Pattern A and Pattern B, until you can do them in your sleep (in fact, I found myself dreaming A & B). But they are the absolute foundation for being able to handle the airplane in the soup, and if you can't do them well, you will have a heckuva time doing approaches well. So don't complain if your instructor bores you with repetitive A & B exercises--they are worth it, for sure. I feel so strongly about this, that I advise if your instructor tries to start you out by shooting approaches, get a different instructor!
Regarding GPS's, I'm still very weak on that, since I've only had my 430W for a little more than a year. With each IPC since, we've spent a lot of extra time on it, because my IPC CFII is a wizard with that box. The real problem for me is that it can do too much, and it's hard for me to remember all the ways to get it to do all that stuff. But having both the equipment and the knowledge to get into just about every airport in the country that has a GPS/RNAV approach makes it worth while.
As for how long it will take you, that's such a variable that it's hard to predict. The more you fly, the fewer hours it's likely to take--that's the same rule for all advanced ratings and certificates. I have known others who have taken the 3 day cram courses; some liked them, some didn't, so I have no advice on that.
But in the end, you'll be a much better pilot for being instrument rated. The insurance companies regularly charge lower premiums to IR pilots, because they know that, too. You'll be much more confident, and you'll have to scrub fewer flights due to weather. But in return, your decision-making will be more difficult, because you'll have more to consider when it comes time to use the IR to make those flights.
Cary