Resources
These are some textbooks and resources that enriched my studies and I recommend them for anyone interested in the respective topics. Click to view and access.
 Real Analysis

Introduction to Real Analysis : Bartle, R.G., Sherbert D.R

Principles of Mathematical Analysis by Walter Rudin

Real Analysis by H.L. Royden and P.M. Fitzpatrick
 Linear Algebra

Linear Algebra Done Right by Sheldon Axler

Introduction to Linear Algebra by Gilbert Strang

Linear Algebra by Stephen H. Friedberg, Arnold J. Insel, and Lawrence E. Spence

Linear Algebra by Hoffmann and Kunze

3Blue1Brown's Linear Algebra Playlist
 Probability
 Complex Analysis
 Ordinary and Partial Differential Equations
 Linear Models

Regression Models by Knieb

Linear Regression Analysis by Douglas C. Montgomery, Elizabeth A. Peck, and G. Geoffrey Vining

Linear Algebra and Linear Models by RB Bapat
 Time Series Analysis
 Machine Learning

Machine Learning and Pattern Recognition by CM Bishop

Introduction to Statistical Learning by Gareth James, Daniela Witten, Trevor Hastie, and Robert Tibshirani

Elements of Statistical Learning by Trevor Hastie, Robert Tibshirani, and Jerome Friedman

Deep Learning Book by Ian Goodfellow, Yoshua Bengio, and Aaron Courville

Bayesian Data Analysis by Gelman

Basic Econometrics by Damodar N. Gujarati
 Causal Inference

Causal Inference in Statistics: A Primer by Judea Pearl, Madelyn Glymour, and Nicholas P. Jewell

Mostly Harmless Econometrics by Joshua D. Angrist and JörnSteffen Pischke

Causality: Models, Reasoning, and Inference by Judea Pearl
 General

Options, Futures, and Other Derivatives by John C. Hull

Fixed Income Securities by Bruce Tuckman

Stochastic Calculus for Finance II by Steven Shreve

The Concepts and Practice of Mathematical Finance by Mark S. Joshi

Stochastic Differential Equations by Bernt Øksendal

Introduction to Stochastic Calculus by Rajeeva L. Karandikar & B. V. Rao

Handbook of Financial Time Series

Quantitative Trading by Ernst P Chan

Basic BlackScholes: Option Pricing and Trading by Timothy Falcon Crack

Inside the Black Box: A Simple Guide to Quantitative and HighFrequency Trading by Rishi K. Narang

Quantitative Primer

Problem Solving Strategies by Arthur Engel

Graph Theory by Douglas West

Mathematical Puzzles by Martin Gardner

50 Challenging Problems in Probability by Frederick Mosteller

Mathematical Puzzles by Peter Winkler

Heard on the Street by Timothy Falcon Crack

Practical Guide to Quantitative Finance Guide by Xinfeng Zhou

Websites

Coffee Can Investing by Saurabh Mukherjea

Bernoulli's Fallacy by Aubrey Clayton

Mathematical Mechanic by Mark Levi

My Life as a Quant by Emanuel Derman

Fooled by Randomness by Nassim Nicholas Taleb

The Black Swan by Nassim Nicholas Taleb