There are a lot of books on writing out there, and a lot on this list. Our advice:  acquire one or two (or more!) in your areas of interest and read widely in your favored forms.

Getting Started, Overcoming Writer’s Block, Finding a Subject
Room to Write, by Bonni Goldberg, has lots of great prompts
Bird by Bird, by Anne LaMott, contains wise counsel on getting it done

Essay and Memoir
To Show and To Tell: The Craft of Literary Nonfiction, by Phillip Lopate
Good Prose: The Art of Nonfiction, by Tracy Kidder and Richard Todd
Anthologies with helpful introductions – for example:
The Art of the Personal Essay, ed. Phillip Lopate
The Best American Essays, College Edition, ed. Robert Atwan

The Lie That Tells a Truth, by John Dufresne
Steering the Craft, by Ursula K. Le Guin
Self-Editing for Fiction Writers, by Renni Browne & Dave King
Anthologies with helpful introductions – for example:
Best American Short Stories of the Century, ed. John Updike
The Norton Anthology of Short Fiction, 2nd ed. – shorter

The Poet’s Companion, by Kim Addonizio and Dorianne Laux
How Does a Poem Mean?, by John Ciardi
Poetry Anthologies – for example:
Good Poems, ed. Garrison Keillor
Norton Anthology of Poetry

Grammar, style, etc.
The Handbook of Good English, by Edward D. Johnson
100 Ways to Improve Your Writing, by Gary Provost

On Writing, by Stephen King
How to Write Funny, ed. John B. Kachuba
Reading Like a Writer, by Francine Prose