Very interesting find, DevilDoggNC! But perhaps we were confused about the terms "Extreme Challenge" (one problem set) and "Math Challenge
" (a compilation of problem sets). Thanks for the clarification.
No problem! I used the "click to look inside" feature on Amazon to look at a few pages. Sackmann has 3 levels of problems -- Level 5 (560-620 level), Level 6 (620-720 level) and Level 7 (720+). It seems like a very comprehensive book -- but the price is a bit on the steep side ($150). I am also told (from sources on this and other forums) that the Manhattan Advanced Quant book is also very good and only costs $ 15.