I think Rees deserves a lot of credit for Ian Book's development from a 3 star to breaking a lot of ND records. He also deserves credit missing on Buchner/passing on JJ McCarthy, so it's a mixed bag, to be fair.
However, I think more of the blame should be on BK's shoulders. During his 10+ years here, every QB except for Book regressed, and regressed noticeably, in year 2 in his system.
The reason I give Rees credit for Book is that Rees was OC/QB coach when Book finally maintained a somewhat consistent level of play from 2018-2020.