From a Geometrical Point of View: A Study of the History and Philosophy of Category Theory (Logic, Epistemology, and the Unity of Science) by Jean-Pierre Marquis
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I wonder if the conceptual arguments could have been made without such mathematical depth, but this is a volume I'll come back to as I learn more. Marquis connects deep ideas that students may be exposed to first formally, without such lucid and farsighted motivation.
One of the best introductions to category theory available. Other sources are more concise (e.g., Mclarty, Goldblatt) but, if you would like to approach category and topos theories from a conceptual perspective, Marquis provides an excellent alternative.