With all due respect to @pgbovine, it is intended as a spiritual successor to both, but is also its own thing. It is being built atop the design methodology of HtDP, which SICP sorely lacks. It has some topical overlap with SICP. But it has an entire second textbook contained in there about the study of programming languages, which neither SICP nor HtDP has. Hence the title: P _and_ PL. It's two books in one, but (increasingly) intertwined.