I'm looking for a math books that you consider that are beautifully designed, it may be by the fonts and the colors used, the structure of the content, equations with a beautiful font, plots beautifully made with nice colors, etc.
Let's say I'm not expecting the traditional LaTeX design. They may be written in LaTeX, no problem with that, but the style has to be different.
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Surely Francis's A Topological Picturebook has to be in the running, right? It has (what I feel is) good design in its typesetting and styling. But obviously its main selling point is the sequence of spectaucular illustrations:
I hope this helps ^_^
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Well, I love Visual Differential Geometry and Forms of Tristan Needham. Its writing and its pictures are wonderful.
On the other hand, I suppose you know The Road to Reality, of Roger Penrose. I think it's a bit overrated, but I admit that the design is really good.
I don’t know any math book that’s as aesthetically pleasing as An Illustrated Theory of Numbers by Martin Weissman. This includes the typesetting, layout and diagrams. You can see some examples of diagrams from the book on that page. One indication how excellent the diagrams are is that the page sells “art prints, adapted from images in the book” :-).