The Most Advanced Clarinet Book -- Austin Macauley publishers.
PDF samples at: austinmacauley.com/author/heimer-tom
(click on the book image then on PDF).
Or check out the book website at: tomheimer.ampbk.com/
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In this 49 page method book there are 50 short studies addressing many technical problems. Material I have discovered over many years and which I presented as a festival clarinet clinician. They are followed by 27 full length contemporary etudes which make use of the short studies as well as basically the most difficult stuff I could think of. After maybe 6 years I could finally play it all myself at reasonable tempos. Is it the "Most Advanced" book? I'll just say that I've played through most of the contemporary Étude books and haven't found one that's harder.