Hi Milo (maybe I should call you Rob)
The Tristan trilogy is some of Messiaen's most secular works dealing as it does with erotic love. Written immediately post war, it consist of the Harawi song cycle, the Turangalila Symphony with solo parts by piano and the characteristic swooping glissandos of the ondes martenot (an early electronic keyboard) and Cinq Rechants, a choral work scored for twelve voices. I had the opportunity to see the Turangalila performed by the SF symphony under M. Tilson Thomas a couple years ago. It was awesome. There a lot of recordings available, a recent English one is said to be really good. My favorite of the ones I've heard is the Deutsche Grammophon one conducted by Myung-Whun Chung with the Orchestre de l'Opera Bastille and Messiaen's wife and sister-in-law as the soloists.
Much of his other work has religious themes with titles like Vingt Regards sur l'Enfant Jesus, Visions de l'Amen, Petites Liturgies de la Presence Divine, Et Exspecto Resurrectionem Mortuorum, you get the idea I think. Among his later large form works, his last piece written in the late 80's, early 90's, Eclairs sur l'Au-Dela (Illuminations of the Beyond) stands out. I again have the DG one with the same conductor and orchestra as above. btw his organ works alone run to 6 or 7 cds of material, as he was for around 60 years the house organist at La Trinite in Paris. The Gillian Weir ones from the mid 90's seem to be considered the definitive versions from what I can tell. I haven't heard them, myself.
Despite the religious themes, Messiaen's work is primarily instrumental so not understanding French won't be any great hindrance in most cases.