I just started working on my Altissimo register in February and the first book I bought after much advise was Rascher's Top Tones. This book sort of pioneered the Altissimo register and has helped many a player learn and facilitate it. After a while though I began to notice some differences. For instance Rascher's Saxophone didn't have a front F or a Palm F or a high F# key, which most modern professional saxes have. I also noticed how many players especially the ones on this forum who worked out of Rascher's book always hit some sort of Barrier. Don't get me wrong I love Top Tones. IMO though I have found a more efficient book Robert Luckey's "Altissimo For the Contemporary Player." IMO this book has been great with no barrier to speak of, and unlike Top Tones it gives varied exercises for overtones not just one.