I would have read the book around 2000 to 2005. I have very faint memories of it so these details may be wildly off: Portrait orientation, at least 6 inches by 9 inches or so. Illustrations were colored line drawings, realistic proportions. There was at least some small element of interactivity - pop-ups, flaps, etc.
The premise was a murder or attempted murder mystery - the setting a medieval castle. The victim has been found (or found themselves) pushed from a height, perhaps into a well. A detective character, perhaps the steward of the castle, is tasked with reconstructing events, interviewing suspects/witnesses. The reader may have been able to validate the suspects' alibis by looking for visual clues in the illustrations, identifying a liar and who was at fault.