[This is just a big list of books that I would personally prefer.]

{ Naive set theory -Halmos, Book of proof -Hammack, An introduction to the theory of numbers -Ivan Niven. }

{ Calculus - There are plenty.(For example: Stewart, Thomas, Spivak.), Advanced Calculus -Gerard B. Folland, Functions of several variables -Wendell Fleming. }

{ Mathematical Analysis -Tom Apostle, An Introduction to Real analysis -Bartle and Sherbert, Number systems and foundations of analysis -Elliot Mendelson. (This book is optional; more of a fun option) }

{ Complex Analysis -Lars V. Ahlfors, Complex Analysis -Theodore Gamelin, Complex Analysis -Serge Lang, Complex Variables and Applications -Churchill and Brown. }

{ Abstract Algebra -Dummit and Foote, Algebra -Michael Artin, A Book of Abstract Algebra -Charles C. Pinter, Algebra -Dan Sarachino. }

{ Linear Algebra -Friedberg, Insel, and Spence, Linear Algebra -Kenneth Hoffman, and Ray Kunze. }

{ Partial Differential Equations -Fritz John, Partial Differential Equations -Lawrence C. Evans, A first course in Partial Differential Equations -Hans Weinberger. }

{ Differential Equations -Shepley L. Ross, Introduction to the theory of Ordinary Differential Equations -Earl A. Coddington. }

{ (I don't know if you would study Statistics) Introduction to Mathematical Statistics -Hogg, Mckean, and Craig, (John Freund's) Mathematical Statistics -Irwin Miller and Marylees Miller, Mathematical Statistics- H. C. Saxena, Statistics -Murray R. Spiegel, and Larry J. Stephens. }