... to go about it - so systematically, I mean: Stendhal’s Four Kinds and Seven Stages of Love.
Nigel's novel sounds interesting, though. Tchaikovsky and Madame von Meck seem to have had a passionate epistolary relationship. And Victor Cousin seems to have fallen in love with Madame de Lafayette, even though she had been dead for a couple of centuries. Ortega y Gasset's On Love is a must-read on this topic.