Reddit mentions of Handbook of Data Structures and Applications (Chapman & Hall/CRC Computer and Information Science Series)

Project Euler is great, but I can't imagine a less efficient way to learn about "advanced algorithmic techniques". Most of the problems are concerned with brute force search, dynamic programming and "clever" ways to trim the computation trees of those two techniques on specific problems.

A better way would be to read books on general algorithms, on optimization, on AI and on machine learning. To that end, two of my favourites are

• The Algorithm Design Manual. The ADM has two parts. The first part is a very good undergraduate algorithms textbook. The second part is a whistlestop tour of as much of modern algorithmics as the author could cover.
  
• How to Solve It: Modern Heuristics. This rolls parts of an AI, an optimization and a ML textbook into one and takes a very practical perspective.
  
  Alternatively, if you've got a specific area of interest then you might want to find a handbook for that area. For example, here's one on data structures.
  
  As a side note, if the price tag of any of those books puts you off, take a look at Library Genesis.

Advanced Data Structures and Handbook of Data Structures and Applications are both well regarded.