There is a wide range of skills that are useful regardless of what variety of poker you want to play, but for any particular game certain skills could be much more relevant than others. A classic example is the ability to successfully determine when a player is bluffing, which is useful in limit holdem but much more important in no-limit holdem. Employing squeeze plays, estimating implied odds, raising effectively for secondary reasons (and recognizing when you opponents might be doing this), and semi-bluffing are a just a few of the many other skills that differ in importance from one poker variety to another.
Sometimes, if I only play one type of poker for any stretch of time, I will completely neglect the skills that are less important for that game. By playing or studying other varieties of poker (or toy games), I often manage to remind myself of some of these other considerations. Not only does this provide an opportunity to make subtle improvements in my primary poker variety, but, for me, it's also a good way to rekindle some excitement about the game by opening up a new direction to explore. This was the main benefit I found to reading a book analyzing Jacks-or-Better poker. A side benefit was that I gained some historical perspective on the history of analyzing poker using game theory.
The book I finished reading was Poker Strategy by Nesmith C. Ankeny. Well, I skimmed it, anyway. That is, I skimmed the parts I didn't skip over. In some ways, the book is horribly dated - imagine if someone today tried giving an unspecific title like "Poker Strategy" to a book theoretically analyzing Jacks-or-better draw poker, which nobody even plays anymore - but much of it was timeless. The game theory analysis looked accurate and went deep enough to provide specific results related to draw poker that I imagine would be extremely useful if anyone still played it.
As I alluded to, there was one skill in particular that was clearly of utmost importance in Jacks-or-Better that is not as central to no-limit holdem: the ability to pin down exactly what sort of hand your opponents are looking for. Given a player's bets before the draw and the number of cards he draws, it is possible to significantly narrow down his range of hands with a high degree of confidence. In particular, you can usually tell if he has a flush draw as opposed to a made hand (or at least a bluff of a made hand). In holdem, it is much easier to disguise flush draws as made hands, and so figuring the probability your opponent holds one type of hand or the other is not nearly as determinative to holdem strategy. (It is relatively useful in stud games, however, and this was my most glaring weakness when I played that form of poker.) As a result, I have not spent much time thinking through the intricate tactics of playing when you have various opponents whose hand types are reasonably well known.
Let me give you an example. In any form of poker, if you strongly suspect your opponent either has a very strong hand or nothing (a drawing hand), it is usually correct to check to him, assuming your hand strength falls in between those extremes. Now suppose you have two opponents, the first of whom likely has a moderately strong hand and the second of whom has one of these drawing hands. This is an interesting situation that I hadn't considered carefully before because it simply doesn't conform to the way I usually think about no-limit holdem, my preferred form of poker. However, Ankeny has a nice, thorough analysis of this situation in his book. It turns out that this situation presents a strong bluffing opportunity if you are betting first, because the player with the moderately strong hand will find it very difficult to call, while the player drawing to the flush or straight will usually miss. I don't know if this situation will come up in any future game I play, but I might find myself in a similar situation, and at the very least it got me to think about the game in a way I might not have otherwise.
Much in the same way the Chen and Ankenman's Mathematics of Poker was useful to me even though it mostly analyzed toy games, the fact that Ankeny's analysis in Poker Strategy was sound and thorough made it somewhat worthwhile even though it did not cover any of the specific games I play.