It's funny how many writers struggle with the simple act of stringing together words in a cohesive, smooth-flowing, clear sentence. The creative part of creative writing is usually the easiest; it's the nitty gritty that proves the greatest challenge!
I've wrestled with far too many sentences to count, as I'm sure most writers have. It can be pretty tough to know exactly how to say what you want to say, in as few and precise words as possible. Thankfully, I've come across some pretty excellent resources for polishing my sentence structure:
The Elements of Style -- This is considered the "Holy Bible of Creative Writing" by many authors. It's a surprisingly concise reference book with all of the rules of writing, and it is a must-have if you want to write properly. As the book says, "one must first know the rules to break them".
Attending to Style -- This is courtesy of Dartmouth University, and it's a resource that will help you to make your writing concrete, active, concise, coherent, emphatic, beautiful, and controlled. It's not very long, but it's VERY helpful!
Purdue Online Writing Lab -- This is a pretty comprehensive resource, with EVERYTHING you could want to know about proper structure, syntax, and grammar. While it's not focused on creative writing specifically, it will definitely help you to clean up your writing.
How to Make Sentences Clear and Concise -- Want to boil down the rules for effective sentences? This resource gives you five simple rules to follow!
Considering Structure and Organization -- Once more courtesy of Dartmouth University, this is a great resource to help you know which sentences belong in which paragraphs--something A LOT of writers get wrong (myself included). The resource is about writing a clear thesis, but it gives you the basic idea of how to write coherent paragraphs. Very handy for creative writing!
Transitional Words and Phrases -- Not sure how to transition between sentences or parts of a sentence? This list of transitional words and phrases (but, yet, beyond, after, etc.) will help you get it right.
Using these resources, you can make your sentences, paragraphs, and chapters smooth and well-structured, leading to MUCH better books overall!