- Practicing your arithmetic! https://arithmetic.zetamac.com/.
- Sequences: https://oeis.org/
- An overview of what to do: https://www.quora.com/What-is-the-best-way-to-prepare-for-quant-interviews-in-finance-at-the-undergraduate-level
- Guide to Competitive Programming: Learning and Improving Algorithms Through Contests: A very good overview of algorithms that are crucial for solving problems fast and efficiently. If you're interviewing for Quant or Backend roles, this book is a must. If you're younger like a high schooler, this book is gold for studying for USACO or ICPC and other contests, but can be hard without having being exposed to some of the CS contests.
- System Design Interview – An Insider's Guide: A great overview about how to solve system design problems for IC4+ roles in the SWE industry. These are problems that involve creating higher-level designs for large systems such as products like Google Drive, YouTube, tinyurl, etc.
- The Art and Craft of Problem Solving: Focused on competition math, but contains lots of good problem solving techniques for you to keep in mind when you get stuck so you can unblock yourself.
- Quant Questions: This site has a large set of problems that you can attempt and keep track of progress (I am not sponsored by them)