This is a recent book and I have not completed reading it but I know already is it destined to become a classic. If you are serious about learning how to understand and improve code, this is the book for you. (Full disclosure: Dick is a long-standing friend and one of the brightest and insightful guys around.)
Writing Efficient Programs (Prentice-Hall Software Series) 1st Edition by Jon Louis Bentley (https://www.amazon.com/Writing-Efficient-Programs-Prentice-H...)
This book is out of print but I believe it's a better read that Jon's Programming Pearls books. C is a great (and permissive) programming language. Jon shows how the language can be exploited for gain performance. Evil but effective.
Hacker's Delight 2nd Edition by Henry Warren (https://www.amazon.com/Hackers-Delight-2nd-Henry-Warren/dp/0...)
The Hacker's Delight is exactly what Hank Warren intended--a collection of tricks and facts that programs can exploit. It's informative on many levels. It uses deep knowledge of how numbers (and other mathematical objects) work to compute useful information. And it shows how seemingly arcane information can be useful. For a programmer excited by the fabric of programs, it is a continual delight.
The first edition has a clarity and compactness that I find appealing. The second edition has additional material. When you get through all of the Delights you can read Knuth Volume 4 which embeds more arcane and useful knowledge.
https://www.amazon.in/Understanding-Software-Addison-Wesley-...