Shrunk and White of Programming
When you put down that you know a certain programming language or languages on your resume, you are setting certain expectations for the interviewer. I would strongly caution against putting down "expert" in a language unless you invented or are one of the language's maintainers. You are giving your interviewer the license to quiz you on programming language lore. There are a handful of concepts that are considered "standard" knowledge for each language which go broadly beyond syntax and general semantics. These concepts commonly involve major pitfalls in a given language and the idiomatic technique for negotiating these pitfalls and writing efficient and maintainable code. Note, although the concepts are considered idiomatic, you can seldom infer them from knowledge of syntax and semantics alone. The tricky part here is that most courses that teach a particular programming language do not cover these idiomatic techniques and even when they do, they do so only in passing. So, it is quite possible that you've taken a course or even multiple courses in a programming language but haven't been truly exposed to these idiomatic techniques that have become the practicing lore in industry. The goal for these books is not to introduce the language to a beginner but rather to inculcate the lore from practitioners on how to write maintainable code that avoids language-specific pitfalls and to be productive in the given language. In other words, these are the concepts interviewers use to distinguishes people who just put down a language on their resume but don't really know the language from those who do. These are also the books that have a good amount of reference value so even after 10 years of active programming, you may still find yourself revisiting and reviewing these books. In this sense, they are the Shrunk and White of the particular programming languages each addresses.
Effective C++: 55 Specific Ways to Improve Your Programs and Designs by Scott Meyers is a classic yet it remains relevant for any C++ programmer or person who is preparing for a C++ interview. The book is organized into 55 items, where each item is a specific concept that addresses a common pitfall in C++ and how to best negotiate that pitfall. Concepts and C++ patterns that Meyers outline such as RAII (Resource Acquisition is Initialization), PIMPL (Pointer to Implementation), and others are standard fare for good C++ programming. Meyers underscores the need to watch out for exception-safety and thread-safety since C++'s rich feature set makes this particularly tricky. A bonus is that this book is quite succinct while carrying along a good number of helpful illustrative examples of each of the specific 55 items. One Uber engineer I know said if she had to recommend one book to prepare for tech interviews, this would be it. Of course, this would refer mainly to companies which use C++. I find myself revisiting this book every now and then just to refresh myself. Meyers now has a sequel to this book called Effective Modern C++, which covers C++11 and C++14 pitfalls, but that book in no way replaces the classic Effective C++. Effective Modern C++ is alright too, but I don't think it is covered as heavily as Effective C++.
Effective Java follows the style of Effective C++ by organizing by items of which there are 78 here. The items are grouped into 11 chapters covering everything from items related to construction and destruction of objects to serialization. Definitely don't put down that you are an expert in Java before you have these concepts down cold.
For Python, Fluent Python is the best book to dive into to get a feel for what experienced practitioners pay attention to. Again, this book isn't for the beginner, but it is for someone who put down that they are proficient or even advanced in Python. This book covers how Python works under the hood and advanced features of Python such as Python metaprogramming with decorators, dynamic attributes, and descriptors. If you want to know how Python arrays, dictionaries, and classes work underneath the hood to prove that you are proficient and actually understand Python, this book will help you get there.
Eloquent Ruby is the book for Ruby.
Disclosure: Yes, those are Amazon affiliate links.