The Elements of Style (William Strunk Jr.)

This classic book on writing and style is a straight-forward guide on writing well. At 105 pages, it is perfect for writers in a hurry (the authors are very cognizant of their readers time) and, for those on a budget, I have the book's Project Gutenburg link below, where it is free to read.

https://www.gutenberg.org/files/37134/37134-h/37134-h.htm