"Drawing Down Belial" by S. Connolly

This book is inspired by goetic tradition and goes into great detail about how to engage in spirit divination.
While this work is more geared towards demonolatry, the general practices work for necromancy.

"A Modern Goetic Grimoire" by Rufus Opus

This book goes into further detail about how the goetic tradition can be practiced in the modern age.
It demonstrates this approach with the infamous "Ars Goetia" from the Lesser Key of Solomon.
The general approach can be applied to a variety of other goetic and necromantic grimoires of the past.
Opus also provides details about the chthonic plane that is highly relevant to the aspiring necromancer.

"Geosophia" by Jake Stratton-Kent

This series of books goes into detail about the relationship between necromancy and goetic tradition.
It is a great resource for approaching older ceremonial grimoires.

"Ars Goetia: The Initiated Interpretation of Ceremonial Magic" by Aleister Crowley

This also focuses primarily on the Ars Goetia from the Lesser Key of Solomon, much like the above two books.
It provides a framework for all of ceremonial magic and Western spirit work in particular.
Alongside "The Psychonaut Field Manual," this should be required reading for any occultist, in my opinion.

General Recommendations for Western Occultism

"The Book of Grimoires: The Secret Grammar of Magic" by Claude LeCouteux

A general overview of grimoiric tradition.

"Witchcraft in the Middle Ages" by Jeffrey Burton Russell

A general historical overview of Western magic.

"Three Books of Occult Philosophy" by Cornelius Agrippa

An influential body of texts that influenced occult tradition.
Long before Eliphas Levi, Samuel Lidell MacGregor Mathers, Helena Blavatsky, Dion Fortune, Israel Regardie, or Aleister Crowley.
Agrippa is arguably as important as John Dee and Paracelsus to the Western occult canon.

"The Psychonaut Field Manual" by Bluefluke

A cartoonist's general how-to guide on performing various kinds of magick. It's free.
If you only ever read one work on magick or occultism, I think it should be this one.
If you're new and need an introduction to occultism, read this with Crowley's "Initiated Interpretation of Ceremonial Magic."

Other authors influential to the occult and New Age movements:

Plato
Aristotle
Plotinus*
Pythagoras
Paracelsus
Hermes Trimegistus*
Nicolas Flamel
Basil Valentine
John Dee
Jakob Bohme*
Emanuel Swedenborg*
Franz Mesmer
Phineas Quimby*
Eliphas Levi
Arthur Edward Waite
William Robert Woodman
William Wynn Westcott
Samuel Lidell MacGregor Mathers*
Swami Vivekananda
Helena Blavatsky*
Alice Bailey
Dion Fortune*
Israel Regardie
Austin Osman Spare*
Rudolf Steiner
Carl Jung*
Gleb Botkin
Franz Bardon
Maharishi Maresh Yogi
Gerald Gardner
Oberon Zell
Linda Goldman
Ram Dass*
Timothy Leary
Terrence McKenna
Robert Anton Wilson
Philip J Carroll*
Ray Sherwin
Phil Hine
Damien Echols
*=These are, in my opinion, the central figures that are the most important for understanding occultism and the New Age movement