Books and articles on magick did not jump into being starting with the 1930s with Gerald Gardner's books on witchcraft.  Occultism and magickal spiritual topics have long been explored in print.  And these days with the internet at our disposal it's increasingly easier to find the old books in print and in electronic formats where before you had to be lucky enough to find them in dusty libraries scattered around the world.

One of the premiere websites that present an archive of old (and therefore public-domain) religious, spiritual, magickal, and occult texts is http://www.sacred-texts.com   They cover the whole realm of religious material but have a special emphasis on occult texts both ancient and modern.  If it's in the public domain chances are you'll find it here.

The Twilit Grotto Esoteric Archives at http://www.esotericarchives.com/ have a good collection of Western ceremonial magick and alchemical texts.

The Hermetic Library at http://www.hermetic.com/ has a wonderful selection of esoteric texts, including an almost-complete collection of Aleister Crowley's libri.

There are a number of publishers who specialize in reprinting old occult texts, and in some cases also publish new texts too.  Check out the catalogues at:

http://www.oakmagicpublications.com/ (mostly English folklore books)

http://www.kessinger.net/ (lots of occult books from the 1800s)

There are also some smaller-press and print-on-demand offerings of specific texts available through online booksellers such as Amazon.  I like to look for used copies through http://used.addall.com/ while new copies are usually available through http://www.bookfinder4u.com/ which searches dozens if not hundreds of online vendors.

If a particular book that you are looking for is not available in an online version and you are not wishing to purchase a copy from a reprint publisher, or you just don't want to own a copy yourself but still want to read it in printed form, be sure to check out your local libraries.  While many libraries won't carry these harder-to-find books, many will happily bring specific books in for patrons using inter-library loans.  In some libraries it's a free service while in other libraries there is a modest fee.  All you need to do is ask your librarians!