I think it is still relevant for low to mid stakes players. However, since you bought our newest sort of comprehensive guide to the game (Crushing Hyper Turbos), going for something like Coffeeyay's Math Pack, or Hokie's Class Pack or Will Tipton's Book + Pack might be ideal. The first breaks down essential math into an easy to understand explanation (and also covers a great deal how to use poker software and how to study to improve your game). Hokie's Class has a lot of population tendency work, how to play from the first hand, how to adjust over time. And Will Tipton's book/pack has a lot more theory and technical study in the game.
Which one you choose really depends on what you're looking to learn. I really like Coffeeyay for someone who is willing to put the time in to improving their play outside of gameplay. Hokie's pack is good if you're really struggling vs unknowns and Will's book/pack is excellent for those that really love the technical side of heads up poker.