My favorite writing resources
While there are the usual suspects (Save the Cat, Steven King’s On Writing), I personally haven’t read them and have found help in other sources.
For plot development, The Coffeebreak Screenwriter by Pilar Alessandra. Yes, it’s for writing a screenplay, but the breakdown of plot is easy and helpful.
On conflict: https://prezi.com/m/uxwk00ijrfib/mastering-conflict/
On romance plot in particular: http://jamigold.com/2012/11/write-romance-get-your-beat-sheet-here/
For grammar, anything by Bryan Garner.
While www.thesaurus.com is helpful, I also LOVE power thesaurus: https://www.powerthesaurus.org/powerful
For description: http://www.obsidianbookshelf.com/html/eyecolorlist.html and http://writersinthestormblog.com/2014/07/writing-about-hair-the-thick-and-thin-of-descriptions/
For style, I’ve found more help in reading articles online than any particular book. My own advice is to avoid the word “was” or “is” because most of the time it’s boring.
Hope this helps!