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The last I'll describe is the fourth book, The Alchemaster's Apprentice, which I haven't finished as I lost my copy and am currently looking for it.
The story takes place in Malaisea, a disease ridden city full of witches, and ruled over by the Alchemaster Ghoolion. The main character is Echo, a Crat, a rare type of talking cat, who's starving to death on the streets after his owner dies. It begins with Ghoolion making Echo an offer, instead of dying of streets Echo can agree to let him use his remains(Crat's fat and organs being potent magical ingredients) to create a potion for immortality and in exchange Ghoolion will feed him gourmet food for the next month before killing him. One difference between this and the others is in the preface, whereas some of the previous books are described as if they're autobiographies of the main characters, this book is actually framed as a novel written by Optimus Yarnspinner, the main character of the previous book.