 The second book, Baldwin 2018, presents mathematical model theory of the period from 1970 to today as a source of material for the discipline of philosophy of mathematical practice.  This discipline studies the work of particular mathematicians within their historical context, and asks such questions as: Why did this mathematician prefer classifications in terms of X to classifications in terms of Y? Why did this group of mathematical researchers choose to formalise their subject matter using such-and-such a language or set of symbols?  How did they decide what to formalise and what to leave unformalised?  The discipline is partly historical, but it looks for conceptual justifications of the historical choices made.  (See the entries mathematical style and explanation in mathematics.) Baldwin has a long history of work in mathematical model theory, so he can answer questions like those above from personal knowledge. This book gives a rich supply of examples, explained with helpful pictures and remarkably little technical notation.