Elementary Algorithms by Xinyu LIU.
From the github page:
AlgoXY is a free book about elementary algorithms and data structures. This book doesn’t only focus on an imperative (or procedural) approach, but also includes purely functional algorithms and data structures. It doesn’t require readers to master any programming languages, because all the algorithms are described using mathematical functions and pseudocode.
For reference and implementation purposes, source code in C, C++, Haskell, Python, Scheme/Lisp is available in addition to the book.
The contents of the book are provided under GNU FDL and the source code is under GNU GPLv3.
The PDF version can be downloaded from github: https://github.com/liuxinyu95/AlgoXY/blob/algoxy/preview/elementary-algorithms.pdf?raw=true
This book is also available online at: https://sites.google.com/site/algoxy/home
I was concerned when the HTML version for trie was only 2 pages long. You need to view the pdf version, which for trie is some forty (40) pages, to get an idea of the coverage of any particular algorithm.
I first saw this in a tweet by OxAX