Middle

C++

Scott Meyers - Effective Modern C++: 42 Specific Ways to Improve Your Use of C++11 and C++14

It's a new chapter of a Scott Meyers books collection. This book contains a collection of advice about C++11/14 standards.
Anthony Williams - C++ Concurrency in Action

This book is a guide for multithreading programming and using the standard library features. It contains detailed explanations about all primitives and their nuances "behind the curtain".
Herb Sutter:
The collection of books describes a lot of tasks related to designing or writing code in C++. They propose a collection of effective solutions. A lot of them are approved and declared as classic idioms. You will regularly find these idioms around projects.
Kurt Guntheroth - Optimized C++: Proven Techniques for Heightened Performance

This book is a guidebook for the performance improvement of C++ applications. Some advice in this book is based on different idioms and tricks described in books of Herb Sutter or Scott Meyers. It's recommended to read this book after reading the books above.

Erich Gamma, Richard Helm, Ralph Johnson, John Vlissides or "Gang of Four" - Design Patterns: Elements of Reusable Object-Oriented Software

This book is a classic guidebook of design patterns. Each pattern is precisely described and advised to a suitable use-case. This book is a good continuation of "Head First Design Patterns" by Eric Freeman. But be ready, this book is more complex than the previous one.
Gary McLean Hall - Adaptive Code

One of the best books that describe SOLID principles in simple terms. All code examples are written in C#, but it shouldn't be a problem as they were written in a simplistic way.
Ulrich Drepper - What Every Programmer Should Know About Memory

This article is a good overview: how PC memory works and why it works in the described way. It shows not only high-level representation, but low-level aspects are also described (if you have an intention to dive in this area).