Good writing consists of good organization and good grammar. There are many textbooks that teach one or the other. In our Level 6 Academic Writing classes, we use two textbooks:
Grammar in Context, Book 3 by Sandra N. Elbaum. This book lives up to its title by providing interesting contexts for the study of the grammatical forms that you need to master in order to become a college writer. There are lots of practice exercises. Previous editions are useful for learners, but only the fifth edition is used in our classes.
Introduction to Academic Writing, by Alice Oshima and Anne Hogue. This book contains a step-by-step approach to writing essays and numerous models and many kinds of practice exercises. We use the third edition in our classes.
Learner-writers also need a good ENNL dictionary; the Longman Dictionary of American English is recommended. Dictionaries written for native speakers do not generally give the grammatical information that learners need.
There are also many web resources for writing improvement. (Watch this page as I add them.)
If you have questions about writing or ideas for how to improve writing skills, email me at firstname.lastname@example.org.