Most writers, including me, have a number of writing guides collected over many years.
The need for guides changes through a writer's career. I started with the beginner guides and, much later, went to marketing and publishing books.
What I was after (and still am) is clear information presented in an entertaining way. I find I recall things better if I like what I've read. Also guides should be encouraging, otherwise why read on?
My favourite writing guide is On Writing by Stephen King. I am also fond of the Wannabe Writer guides written by the much missed Jane Wenham-Jones.
A good test of a guide is its readability (and re-readability).
Which guides have you found useful?