Just to note the arrival at the Stanford Encyclopedia of a new entry ‘Structuralism in the Philosophy of Mathematics’ by Erich Reck and Georg Schiemer.
Two features of this entry are potentially particularly useful. First, there is an attempt to provide a helpful taxonomy of the many varieties of structuralism about mathematics. Second, there’s a discussion of the relation between category theory and structuralism.
There’s quite often a problem of level and speed of coverage with SEP entries. I’d say that might apply here — the piece necessarily goes quickly over a lot of ground, and I’d guess that many a last-year undergraduate/first-year graduate student wanting a way into the topic will find that this entry goes too fast. For just one example, I don’t imagine someone getting much of an idea of Charles Parsons’s position from the brief discussion here. My guess is that an entry perhaps 30% longer, going more slowly over the same material, could have been 100% more useful to many student readers.
However, if you do already know a little about structuralism in the philosophy of mathematics, and want a well organized guide to help you get to know a lot more (and to spur you on, perhaps, to finding out something about category theory!), with very helpful pointers to the literature, then this will be a quite excellent place to start.