For advice on writing, nothing beats Nicolas Boileau, founder of the Académie Française, who wrote a creative-writing course in verse called L'Art Poetique. It dates to the mid-l7th century but it's still the best around. The translation is by John Dryden. Some samples:
Polish, repolish, every color lay,
Sometimes add, but oftener take away.
On not messing up your murder mystery with a love story, and avoiding comic relief:
Remember always never to bring
A tame in union with a savage thing.
On avoiding too many subplots, unnecessary characters, and the urge to put all you know in one book:
Make not your tale of accidents too full;
Too much variety will make it dull.
Achilles' rage alone., when wrought with skill,
Abundantly does a whole Iliad fill.