Books to help improve writing at the sentence level.

I'm a hobbyist writer and I like to find books that could help me beef up my prose. I look at what I have written and feel like I can do better but the problem is I don't know how. I already have self editing for fiction writers but my sentences still feel so flabby. Are there any books I could buy that get down to the nuts and bolts of writing and help you understand sentence structure better?