I just found out that the first edition of the Elements of Style, an excellent 50 pages book about good writing in English, is available for free online. It first covers efficiently the most common mistakes in English and then proceeds to the real nuggets: the elementary principles of composition
The book is available here: www.gutenberg.org/files/371…
I cannot recommend it enough. It improved my writing immensely (although it can still be improved a lot).
If you do not have much time, just read these two principles:
I read the 4th edition, not this first one from 1918 and I have the feeling that it was better formatted, on top of using a more modern language, but on the other hand, you cannot beat free.