General
https://archive.org - a vast trove of public-domain books and other media on an endless number of topics
You'll want to use the search function. For example, try "amateur radio", but more specific topics can produce good results. There are lots of magazines, such as "73 Radio". It may be helpful to sort the search results "by title" and to click the "show as list" button.
Shortcut: a relevant subset of archive.org is the Digital Library of Amateur Radio and Communications.
https://www.gutenberg.org - another repository of public-domain ebooks
Use the search function to identify books of interest. For example, try "amateur radio". This site is not nearly as deep as archive.org, but it seems to be more focused on books than magazines and other digitized media.
https://onlinebooks.library.upenn.edu - more public-domain ebooks on a multitude of topics
Try the subject search for "ham radio", or a specific title or author search. Some results point to other collections, such as archive.org.
General - technical
After navigating to these sites, you may wish to use your browser's "search this page" function (often CTRL+F) to look for titles on specific topics or by a specific author.
https://www.n5dux.com/ham/files/pdf
https://www.iz3mez.it/resources/ebook
Note: both of these sites offer works that may be under copyright.
http://tubebooks.org - info related to vacuum tube technology
The curator of this site seems to have taken care to offer scans of vintage works that are no longer under copyright.
Educational
http://www.ibiblio.org/kuphaldt/electricCircuits/DC/index.html - Lessons in Electric Circuits, Vol. I: DC – open textbook
http://www.ibiblio.org/kuphaldt/electricCircuits/AC/index.html - Lessons in Electric Circuits, Vol. II: AC – open textbook
http://www.ibiblio.org/kuphaldt/electricCircuits/Semi/index.html - Lessons in Electric Circuits, Vol. III: Semiconductors – open textbook
http://www.ibiblio.org/kuphaldt/electricCircuits/Digital/index.html - Lessons in Electric Circuits, Vol. IV: Digital – open textbook
http://www.ibiblio.org/kuphaldt/electricCircuits/Ref/index.html - Lessons in Electric Circuits, Vol. V: Reference – open textbook
http://www.ibiblio.org/kuphaldt/electricCircuits/Exper/EXP.pdf - Lessons in Electric Circuits, Vol.VI: Experiments – open textbook
http://www.arrl.org/ares - Amateur Radio Emergency Service® (ARES) manuals and planning materials
https://www.qsl.net/sp9hzx/img/The%20Beginner's%20Handbook%20of%20Amateur%20Radio.pdf - 4th edition of a work by Clay Laster, W5ZPV - an introduction to amateur radio for US hams
This PDF is from 2001, so it is a bit dated with regard to things like FCC exam hints and the now absent CW requirement to get a license. Additionally, it appears to be in print and can be purchased, new, from Amazon. The site that offers this volume may be in violation of copyright.
Ham Radio Callbooks
https://leehite.org/callbooks - a collection of scanned callbooks from 1906 to 1997. Many are searchable. Also some ARRL Handbooks.
Historical
http://www.astrosurf.com/luxorion/qsl-ham-history.htm
As part of a site located in France covering various topics of scientific interest, here is a fairly detailed overview (in English no less) of the global development of amateur radio from the early days up until 2015. Note at the end are additional links to substantial resources for the Ham operator elsewhere on the internet.
http://www.readcentral.com/book/Norman-F-Joly/Read-The-Dawn-of-Amateur-Radio-in-the-UK-and-Greece-A-Personal-View-Online - The Dawn of Amateur Radio in the U.K. and Greece A Personal View by Norman F. Joly – open e-book
ReadCentral appears to be free but requires that you create an account and log in to use their online reader. This book is also available from Gutenberg.
https://worldradiohistory.com - publications about the history of radio, especially the broadcast industry
This extensive site offers both historical and technical information, with a variety of search functions that are offered for different topic areas. eBooks can be found in the Bookshelf section.
http://openlibrary.org/books/OL7038250M/Principles_of_radio_communication - Principles of radio communication. Published 1921 by Wiley in New York . 935 pp.
You'll use an online reader (not a download), or you can hear the book in audio form. For the reader case, the single page format is convenient, and you can simply scroll down as you read.
Entertainment
http://www.readcentral.com/book/Gerald-Breckenridge/Read-The-Radio-Boys-with-the-Revenue-Guards-Online - The Radio Boys with the Revenue Guards by Gerald Breckenridge –free, online novel
ReadCentral appears to be free but requires that you create an account and log in to use their online reader.
http://archive.org/details/bertwilsonwirele00duff - Bert Wilson, Wireless Operator – a free, online novel
Partial book report (I read the first 50 pages), by WB4SPB:
You'll use an online reader (not a download) to read this book. I recommend single-page format, where you can simply scroll down to see the next page. Make a note of your current page number if you want to put it down and go back later.
Copyrighted 1913, published 1924, these are the adventures of a college student on his summer job as radioman for a shipping vessel in the Pacific. As befits the era, the radio technology is spark, and the descriptive language is nicely sensoral if not technical. Of course, our hero acquits himself handsomely (in the first 50 pages anyway) in what is for him a dream job handling important traffic and maintaining the ship's radio gear.
The narrator's voice is similar to that found in old Hardy Boys novels (think: "By Jove!" the lads expostulated.). There is, of course, much comeraderie among the major characters, who pull together toward that which represents hard work, friendship, courage and similar, high-minded values.
Societal attitudes revealed by the author are also what you might expect from the time in which this book was written. Women are marginal, emotional and frightened; racial stereotypes are pronounced, even lurid. But maybe you can read it quickly and skim over the parts that are offensive to today's moral sensibility. For people who are into radio, there's a yarn in here that takes us back to the spark era; and since that's when it was written, the experience of the protagonist seems fairly authentic.
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