Most likely? Nothing. The book is standalone and most of what you need to know is quickly recapped fairly quickly in the text itself (The book was written that way, and I have had multiple positive reader feedback on It being standalone).

If you still feel lost:

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WARNING: HERE BE SPOILERS

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SPOILERS, scroll down to get there

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STILL SPOILERS, scroll down a little more…

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HERE YOU GO!

The Great Houses, the magical factions of Paris, have suffered a setback when a dark curse engulfed House Silverspires, the first and greatest of Houses (located on Ile de la Cité in the centre of Paris and centred around the ruins of Notre Dame). Silverspires survived but is now much diminished, and still half-covered in the roots of a huge magical banyan tree, the remnants of what almost ended the House.

Madeleine, once Silverspires’ alchemist, was cast out of the House when her addiction to the dangerous drug angel essense was made public. She was a refugee from the neighbouring House Hawthorn: the head of that House, the sadistic Fallen angel Asmodeus, has now claimed her back. Madeleine still lives in fear of Asmodeus, as his people were the ones who slaughtered all her friends and left her for dead in the wake of the coup that got Asmodeus to the leadership of the House, twenty years prior.

Philippe, an Annamite (Vietnamese) and ex-Immortal, has fled House Silverspires, where he was held captive and accidentally unleashed the curse that all but destroyed the House. He now lives in the Houseless areas, and seeks a way to resurrect his friend, the Fallen Isabelle–who belonged to House Silverspires and died in defence of it. Prior to her death, Philippe’s and Isabelle’s relationship had turned fraught: he thought she was had become cruel and ruthless, the same casual cruelty he’d seen all too often from Fallen to justify atrocities.

House of Binding Thorns buying info: