Well, it isn't "lost" of course because you can get it here and no doubt lots of other places. Recorded by the latter-day Crimson, "Double Duo" of Fripp, Gunn, Belew and Mastelotto, it was nonetheless regarded as a side project and attributed to Projekct X rather than to King Crimson proper. I notice Amazon says it's by "King Crassius" - sounds like a Stingray villain - which will no doubt make it even harder for the uninitiated to track down.
I think it's really a great record, freewheeling and sonically adventurous. One of them referred to it as (I think) "Crimson's techno alter ego" and the rhythm section particularly lays down great, heavy, modern-sounding grooves over which we get the usual prog-jazz-metal Crimson sounds, soundscapes as well as some spectacular Fripp flights. The title track is one of the most straightforwardly beautiful in the Crimson catalogue.
Construction of Light sounded like a strange, Pierre Menard exercise in recreating 70's Crimson, much later, for their own reasons. Heaven and Earth, in contrast, is truly progressive and forward-looking; but perhaps in some way incompatible with Fripp's ideas of King Crimson identity and possibly audience expectations. It may be a bit too freewheeling for some. If, however, you feel like me that Crimson's true greatness lies in bringing the approaches and some of the sounds of free jazz to stadium rock, you'll find this a hugely enjoyable development in that tradition.