First the obvious similarities: Both are noir fiction in which a protagonist gets tangled in the plottings of an aged occultist. In both, unusual events unfold naturalistically, though driven by the perversities of strange enemies. Both take place in urban and suburban landscapes hollowed out by recent economic catastrophes: Tokyo after the deflation of the Japanese bubble economy, Long Island in that interregnum before the military-industrial complex learned how to remake profits in the new world order.
There are differences, of course. Where Dawn Seliger is a believer who is made vulnerable by the collapse of everything that underpins her beliefs, Shunichi Otsuki is a cynic of long standing. His vulnerability stems rather from the fact that, having long since hit rock bottom, he cannot imagine that he has still further to fall. Their gender difference also imposes itself: Otsuki, being a heterosexual male, is prone to treat women as MacGuffins, and so the plot uses women as such, whereas Dawn's motivations are more ideological in nature.
I have some slight dissatisfactions with Triangle, the most significant of which is the name. "Spiral" might have been a more accurate rendering of the Japanese title Tomoe, and more consistent with the fundamental pattern of the narrative. There were also some occasional misprints that impaired the sense of the prose--one expects better from Dalkey Archive, and hopes they fix them in future printings. One hopes especially that the book does well enough that there are such future printings.
If you liked Love Is the Law, then you should read Triangle. If you haven't read it yet, then you should read both, especially if, for example, you have read and enjoyed Daniel Alarcón's At Night We Walk In Circles. If you haven't read any of those three books I just mentioned, then what's wrong with you? Hurry up about it!