Here is an arbitrary list of helpful online resources that I am collecting, mostly for my own benefit, listed in no specific order.

• Online Encyclopedia of Integer Sequences

• Introduction to Random Graphs
  textbook by Alan Frieze and Michal Karoński

• Lecture Notes on Entropy and Markov Chains
  by Pietro Caputo (Roma Tre)

• Lecture Notes on Spectral Independence
  by Daniel Štefankovič (University of Rochester) and Eric Vigoda (UCSB)

• Eigenvalues and Polynomials
  lecture notes by Lap Chi Lau (@ University of Waterloo)

• Lecture Notes on Expanders and HDX
  lecture notes by Max Hopkins and Shachar Lovett (@ UCSD)

• Analysis of Boolean Functions
  textbook by Ryan O'Donnell

• Expander Graphs and Their Applications 
  survey by Sholmo Hoory, Nathan Linial, and Avi Wigderson

• Surviving as a Quantum Computer in a Classical World
  textbook by Daniel Gottesman

• The Theory of Quantum Information
  textbook by John Watrous

• Lecture Notes on Quantum Computation
  lecture notes by John Preskill (@ Caltech)

• Quantum Information Science
  lecture notes by Aram Harrow (@ MIT)

• Essential Coding Theory
  textbook by Venkatesan Guruswami, Atri Rudra, Madhu Sudan

• Computational Complexity: A Modern Approach
  textbook by Sanjeev Arora and Boaz Barak 

• Pseudorandomness
  survey/monograph by Salil Vadhan

• High Dimensional Probability
  textbook by Roman Vershynin

• Lecture Notes on the Sum of Squares Hierarchy
  lecture notes by Boaz Barak (@ MIT)

• Log-Concave Sampling
  textbook by Sinho Chewi

• An Introduction to Matrix Concentration Inequalities
  textbook by Joel Tropp

• Entropy Optimality
  lecture notes by James Lee (@ University of Washington)

• Convex Optimization: Algorithms and Complexity
  monograph by Sébastien Bubeck

• Convex Optimization
  textbook by Stephen Boyd and Lieven Vanderberghe

• An Introduction to Modern Convex Geometry
  notes by Keith Ball

• Algorithmic Convex Geometry  
  notes/book by Santosh Vempala

• Mathematical Aspects of Markov Chains and Mixing Times
  monograph by Ravi Montenegro and Prasad Tetali

• Probability on Trees and Networks
  textbook by  Russel Lyons and Yuval Peres

• Reversible Markov Chains and Random Walks on Graphs
  unfinished monograph by David Aldous and James Allen Fill

• Markov Chains and Mixing Times
  textbook by David Levin, Yuval Peres, and Elizabeth Wilmer

• Ten Lectures and Forty-Two Open Problems in the Mathematics of Data Science
  lecture notes by Afonso Bandeira

• Music: a Mathematical Offering
  textbook by David Benson

• Living Proof: Stories of Resilience Along the Mathematical Journey
  collection of essays edited by Allison K. Henrich, Emille D. Lawrence, Matthew A. Pons, and David G. Taylor