Historical Source Material Online

Many texts can be viewed via Google Books. This includes limited (but often useful) previews of copyright texts not available online. Some authors have many more works available online - I have chosen a selection.

Ancient World

The Gnostic Society Library contains an extensive selection of gnostic source texts. The Perseus Digital Library has a huge collection of classical literature. The Internet Sacred Text Archive is a good general source.
The Eleusinian and Bacchic Mysteries, Thomas Taylor [Background] [Text]
Thrice Greatest Hermes, G.R.S. Mead [Background] [Text]
Fragments of a Faith Forgotten, G.R.S. Mead [Background] [Text]
The Chaldean Oracles, G.R.S. Mead [Background] [Text]
Papyri Graecae Magicae [Background] [Text]
Porphyry, Letter to his Wife Marcella [Background] [Text]
On the Mysteries, Iamblichus [Background] [Text]
The Sword of Moses, trans. M. Gaster [Background] [Text]

Judaism

Tanakh [Background] [Text]
The Babylonian Talmud, ed. R. Dr. Isidore Epstein [Background] [Text]
Genesis Rabbah, trans. R. Dr. H. Freedman, [Background] [Text]
Pirke De-Rabbi Eliezer [Background] [Text]
The Guide for the Perplexed by Moses Maimonides [Background] [Text]
Ethiopian Book of Enoch (1 Enoch) [Background] [Text][Text]
Slavonic Book of Enoch (2 Enoch) [Background] [Text]

Jewish Kabbalah

Song of Songs [Background]
Hekhalot Rabbatai, trans. Morton Smith [Background] [Text]
Shi'ur Komah, trans. WoC [Background] [Text]
3 Enoch [Background] [Text]
The Sepher Yetzirah, trans. Aryeh Kaplan [Background] [Text]
The Bahir, trans. Aryeh Kaplan [Background] [Text]
Idra Rabba Kadusha, trans. WoC [Background] [Text]
Idra Zuta Kadusha, trans. WoC [Background] [Text]
Sefer De-tzeniuta, trans. WoC [Background] [Text]
Sefer De-tzeniuta, trans. Pinchas Giller [Background] [Text]
The Kabbalah Unveiled (Idrot), Rosenroth & Mathers [Background] [Text]
The Zohar [Background][Background]
The Palm Tree of Deborah (Tomer Devorah), R. Moses Cordovero [Background] [Text] [PDF]
Works of R. Moses Chaim Luzatto [Background] [Texts]
The Tanya, R. Schneur Zalman [Background] [Text]
Sefer Raziel HaMalakh [Background]

Christian Kabbalah

Selected Writings of Franciscus Mercurius van Helmont, ed. Don Karr [Background] [Text]
The Confessions of Jacob Boehme, ed. W. Scott Palmer [Background] [Text]

Hermetic Kabbalah

The Cloud Upon the Sanctury, Karl von Eckhartshausen [Background] [Text]
The Mystical Qabalah, Dion Fortune [Background] [Text]
The Cosmic Doctrine, Dion Fortune [Background] [Text]
The Tree of Life, Israel Regardie [Background] [Text]
The Middle Pillar, Israel Regardie [Background] [Text]
A Garden of Pomegranates, Israel Regardie [Background] [Text]
Book IV, Aleister Crowley [Background] [Text]
Liber 777, Aleister Crowley [Background] [Text]
The Equinox, ed. Aleister Crowley [Background] [Text]
The Book of Lies, Aleister Crowley [Background] [Text]
The Anatomy of the Body of God, Charles Stansfield Jones [Background] [Text][Text]

Theurgy, Thaumaturgy and Alchemy

The Esoteric Archives contain most of the important theurgic and thaumaturgic works ... except the Necronomicon, which remains in the locked, reserved book cabinet of Miskatonic University.

The Alchemy Website contains just about everything of importance relating to alchemy.

Three Books of Occult Philosophy, Cornelius Agrippa [Background] [Text]
Solomonic Magic, ed. Don Karr [Background] [Text]