Not a dictionary, but a novel written in a series of vignettes documenting the feelings and day-to-day negotiations of a romantic adult relationship. Each diary-like entry is a prose poem titled alphabetically with a noun, verb or adjective: aberrant, abstain, abstraction, abyss, acronym, adamant, etc.
The tale is shaped around the consequences of an infidelity. It is told in first person without names, nor very many clues about the gender of either partner, an ambiguity which reinforces the universal nature of the subject. Levithan even references Jeanette Winterson's famously non-gender-specific novel, Written on the Body in the poem 'blemish, n.' In my interpretation of the text, however, I kept seeing two men. This may be because I know the author is gay and his previous books have had gay central characters. (This is also his first clearly adult novel after a string of successful YA titles.)
Here's an excerpt from the poem 'woo, v': "I told you that it was ridiculous to pay thirty dollars for a dozen roses on Valentine's Day. I forbade you to do it. So that day, when I went to pay for lunch, what did I find? In my wallet, thirty singles, each with roses printed on it."
Not every scene is so sweet, of course. It's a candid examination of the nature of human partnerships. It is absolutely charming.
Readalikes on the subject of love with a similarly unusual format (albeit different tones): The Incident Report by Martha Baillie; The End of the Alphabet by CS Richardson; Watercolor Women, Opaque Men by Ana Castillo; Anthropology by Dan Rhodes; The Beauty of the Husband by Anne Carson.