Pretty sure this book was fantasy, rather than sci-fi. Probably late 70s early 80s. I remember:
The world is made up of separate chunks, islands floating in space, each following a specific path that is charted and regular (there are timetables written about the movement and paths of each island). A thief cases a house to steal an artifact, a crystal crescent. The crescent is resting on a cushion on a mantel, in a room where the floor is specially made so that if you step in the wrong place, the floor shrieks like a riven vampire. The thief returns at night, navigates the floor, discovers that the crescent is protected by a glow (spell), figures out that the spell doesn't affect the silk cushion the crescent is sitting on, so makes a loop out of a piece of silk from his own clothes, and leaves (something) behind on the cushion, to give the spell something to protect. To leave the building, he goes to a pantry/closet with a window, and has to dislocate his shoulder to fit. I want to say that he can shapeshift into a bear, but I can't remember how that fits in anywhere. Further on, he meets a sorceress, who wears a ring made out of crystal on a particular finger to show her rank.
And that's what I remember. I want to say the author is female, but I can't remember the name of the author or the book, and the copy I read was missing its front cover so I have no idea of the cover art. Please help!