A few of my favourites that immediately come to mind:
- Janich, Klaus. Topology
- Carter, Nathan. Visual group theory
- Lawvere, F. William and Stephen H. Schanuel. Conceptual mathematics: a first introduction to categories
- Aigner, Martin; Ziegler, Günter. Proofs from THE BOOK
Also, in their own weird way:
- Steen, Lynn Arthur and Seebach, J. Arthur. Counterexamples in topology
- Nelsen, Roger B. Proofs without words: Exercises in visual thinking (series)