Although I admit to being a writing advice junkie, there are two books that have exerted considerable influence on my actual writing practices and to which I return again and again and one exceptional one that I've recently read.
The first, and most referenced is Andrew Tobias' 20 Master Plots, whose title may mislead you into thinking that this is a cookbook of winning formulae. Instead, Tobias breaks down the essential elements of twenty plot themes; Quest, Adventure, Escape, Revenge, etc. and discusses why they work as they do and provides illustrative examples.
Years ago I was writing an adventure novella (Magic's Price) when I first encountered this book. Upon reading Tobias' chapters, I realized that my story was actually a character study of the maturing protagonist who must face the consequences of his choices.
But personal insights are not where I've found this particular book so useful. I'ver returned to it time and again to check my story, to see if I have missed something, or only to verify my writing instincts. In this latter respect I've used the checklists Tobias provides at the end of each section on the essential elements usually found when using in that particular approach. Including them is optional, but validating your story against them forces you to justify to yourself just why you choose otherwise. I strongly feel that this book is an essential reference for fiction writers at any level, but most importantly for the short fiction writer.
The second book is Nancy Kress' Beginnings, Middles and Ends which is more focused on developing the long form. This book introduced me to the idea of successive layering, adding another level of meaning with each successive edit and, in a blinding revelation ( for me, anyway) that the meaning of a story comes after you finish, no matter what you intended. Kress suggests that the final edit, the layer of varnish that brings focus to the entire work, is the injection of symbolism wherever it makes sense. We often see an iconic item, such as the wonderfully descriptive passages about the train in ian Mcdonald's Ares Express, which while essential to the plot, nevertheless is the touchstone around which all else revolves.
Kress also provides important advice on crafting the story's climax - that essential element that concludes the throughline and gives the reader the satisfaction they deserve. But this discussion comes after many pages of advice that she has codified over her many years of writing experience.
Finally, I cannot be effusive enough about Kate Wilhelm's Storyteller which provides a plethora of writing advice. After a quick review I decided to read it more carefully and put a marker wherever I saw advice I should heed. When I'd completed this slight volume I had almost run out of markers. Wilhelm's writing advice is presented painlessly, along with a bit of sf history and its personalities that makes the entire book fun to read. The real bonus comes at the end, where Wilhelm provides a review in fifteen short (and factually dense) pages. I feel that I will return to this book over and over, if for nothing else than reassurance that my writing agony and frustration is not unique.