The book **Philosophy of Mathematics** by Øystein Linnebo, is a nice, systematic and accessible introduction to the philosophy of mathematics by a leading scholar in the field.

**Plato's Problem: An Introduction to Mathematical Platonism** by M. Panza and A. Sereni.

**A Subject With No Object: Strategies for Nominalistic Interpretation of Mathematics** by John P. Burgess and Gideon Rosen. A book on the strategies of various nominalists in mathematics who have attempted to "purify" mathematics from abstract objects, and a critical assessment of such efforts by the authors.

[SEP][1] and [IEP][2] are good sources of information about philosophy.

[PhilPapers][3] is a comprehensive index and bibliography of philosophy, maintained by the philosophy community.

  [1]: https://plato.stanford.edu
  [2]: https://iep.utm.edu
  [3]: https://philpapers.org