Then there’s the gimmick of binding books with human skin. Also known as Anthropodermic bibliopegy. The Anthropodermic Book Project lists their last update (2019) here.
Philly’s Mutter Museum (there should be an umlaut over the u in Mutter) has several books that are supposed to be bound in human skin. IIRC the display explains that surgeons would train on a cadaver, writing and drawing what they learned in a journal. When the cadaver was all used up, the skin was used to make covers for the journal. Considering the vast amount of human remains and bits in the Mutter, I’m assuming the display is accurate.
In the Encyclopedia Brown series of books the answer to the mystery is printed backwards so you either have to hold the page up to a light and read it from the reverse side or hold it up to a mirror.
As a child I had a picture book that came with a flexible mirror that you used to view “both sides” of the illustrations.
And two obvious ones I haven’t seen mentioned yet are books with “built in” markers (like a bible with a marking ribbon) and diaries with a built in lock.
I have a book of illustrations where each page was divided into two or three parts. Each top half (or third) could be matched with any other illustration to create all kinds of weird pictures.
I’m not sure if this counts as a “gimmick book”, since it’s really the topic that makes it a gimmick.
There is a type of art called “Anamorphic art”, where the original image is distorted, and only properly viewable in a curved mirror (or at an extreme angle), Typical mirrors are cylindrical or conical mirrors.
I have a couple of books on anamorphic art, and both come with a sheet of aluminized mylar that can be rolled into the required cylinder or cone for viewing.
Maybe not a gimmick, per se, but it felt like one:
Stephen King put out two books simultaneously, The Regulators and Desperation, that were “mirror images” of each other – same characters, mostly. It was hard not to think of one while you were reading the other and that was a huge distraction. Plus, they really were not that good in the first place.