Quantitative Aptitude for Competitive Examinations - Fully Solved by R S Aggarwal

 

This book is very easy to overlook if, and that is a big if, you ever find it - BUT YOU MUST USE IT. It might look outdated, and it is not written by an American or an European but an Indian (what might be it's actual high point if you ask me...), but this one is a gold mine I say!
This book takes the correct math approach to solving problems and gives you correct mathematics shortcuts, unlike many others that teaches you shortcuts that might work, but do not contribute to your understanding why it works.