A classic. Literally, since Rutherford Aris was, uniquely, I imagine, an eminent classics scholar, as well as a top class chemical engineer. So a sprinkling of latin tags and untranslated quotes in foreign languages. Beautifully written text about the philosophy of modelling. Not an introduction for beginners.
A very practical approach to modeling physical phenomena (particularly for chemical engineering) using intuition, dimensionless variables, appropriate units, and other strategies to bring the most essential information to the fore. Holds up well over time.
His discussion of certain oscillations in chemical reaction equations was quite new to me.