There is a tremendous amount of information on the internet - the equivalent of thousands of reference libraries. For those who can't easily get to a library, or who cannot afford books, the internet is a wonderful invention. There are many, many books that can be downloaded for free, to be printed out and read at leisure - these include much of the writings of the classical and ancient ages. The list below (compiled with the invaluable help of Kim Farnell) will be of particular interest to astrologers.
Note Most of these texts are old and out of copyright. However, a few works are still in copyright, and will therefore have restrictions on their use; please read and observe these restrictions.

Considerations of Guido Bonatus — www.panplanet.com/library/bonatus
From Humanistic to Transpersonal Astrology by Dane Rudhyar (1975) — www.khaldea.com/rudhyar/fromhtot_1
An Attempt at Formulating Minimal Requirements for the Practice of Astrology by Dane Rudhyar — www.khaldea.com/rudhyar/attempt
A Handbook for the Humanistic Astrologer Michael Meyer http://books.iuniverse.com/viewbooks.asp?isbn=0595089356&page=fm3
The Ancient Greek Sacred Lunar Month © 1999, Apollonius Sophistes — www.cs.utk.edu/~mclennan/OM/BA/PT/BA/SM
The Ancient Greek Esoteric Doctrine of The Elements © 1998, John Opsopaus — www.cs.utk.edu/~mclennan/OM/BA/PT/BA/AGEDE/
The Eightfold Year and the Stages of Life ©1995, John Opsopaus — www.cs.utk.edu/~mclennan/OM/BA/PT/BA/EYSL
PLANETARY QUALITIES an anthroposophical view by Bibi van Bussel — www.xs4all.nl/~busbi
Astro-Diagnosis A Guide To Healing by Max Heindel and Augusta Foss Heindel — www.rosicrucian.com/adh/adheng01
Astrology Theologised Valentin Weigel (1553-1588) www.passtheword.org/DIALOGS-FROM-THE-PAST/weigel
Abu Mashar - Libri mysteriorum, The astrological metaphors (HTML format): translated by Giuseppe Bezza — www.cieloeterra.it/eng/eng.testi.metafore/eng.metafore
Bullinger, E. W. - The Witness of the Stars — http://philologos.org/__eb-tws/
Culpeper, Nicholas - The English Physitian (1652) — www.med.yale.edu/library/historical/culpeper/culpeper
Morin, Jean Baptiste - The Cabal of the Twelve Houses Astrological — www.skyscript.co.uk/cabal
Ptolemy - Tetrabiblos 1 — www.kronosofia.dk/frames/side/biblioteket/tetrabiblos/tetrabiblos
Mnemonic Arts of Blessed Raymond LULL — www.lullianarts.net
Ebenezer Sibly - A New and Complete Illustration of the Occult Sciences, Book 4 — www.esotericarchives.com/solomon/sibly4
Light Of Egypt, The; or, The science of the soul and the stars -- Volume 2 by Burgoyne, Thomas H. — www.gutenberg.net/browse/BIBREC/BR1650
Astrology and The Ductless Glands by Max Heindel — http://fraktali.849pm.com/rosen1
Living In Time by Palden Jenkins — www.isleofavalon.co.uk/time
The Historical Ephemeris from -600 to 2200 Astrological tables and historical timelines demonstrating the relationship between world historical trends/events and planetary movements for the use of historians and astrologers, by Palden Jenkins — www.isleofavalon.co.uk/ephem
The 12 Signs of the Zodiac by C.E.O. Carter — www.digthatcrazyfarout.com/carter/Carter_Leo
History of Astrology by Derek and Julia Parker — www.meta-religion.com/Esoterism/Astrology/history_of_astrology

All the following can be downloaded from www.panplanet.com/library/downloads:
John Gadbury - Doctrine of Horary Astrology
Guido Bonatus - Anima Astrologiae
Sepharial - Primary Directions Made Easy
Sepharial - Astrology and Marriage
Jerome Cardin - Choice Aphorisms
Anthony Griffin - Theft
Considerations on the Fundamental of Astrology - Kepler
Electional Astrology - Vivian Robson.