If you use "Book of Moon" on a "Monarch" monster, the effect still goes through because it does not have to be face-up on the field for the effect to resolve. "Spirit Reaper"s effect is continuous, so it has to be face-up on the field to resolve.
"Tsukyomi" activates as soon as it is face-up on the field and it doesn't need to be face-up on the field to resolve because it's a trigger effect and not continuous (like the "Monarch"s), so he would be Summoned, the effect would activate and a target selected, then the opponent could chain "Book of Moon" to his effect, targeting "Tsukyomi". Resolve backwards, "Book of Moon" turns "Tsukyomi" face-down and "Tsukyomi" resolves and flips the targeted monster face-down.