Good tips on this evergreen topic, and honestly I'd think it would be quite weird if someone
didn't have intonation issues in the upper register, at least for a good long while. It usually takes many years to develop the kind of physiological control required for playing the "top tones" with control.
The book that helped me the most (aside from Rascher) was Dave Liebman's
Developing a Personal Saxophone Sound, which is largely based off of Rascher and Allard, AFAIK. But any of those methods will be helpful. My tone role model for all that is the great Mark Turner, whose altissimo control is always extremely beautiful and effective. His solo on this track is masterful, IMO.
View: https://youtu.be/5q62A-fNmi0?si=fXFejVDrTLhWZH_U