AmericanRadioHistory.com A site hosting a huge library of old magazines and some books. For all that it's American they have a lot of British stuff too — they've got a full set of Radio Constructor and a lot of Practical Wireless/Television/Electronics.
Yahoo! Hacker Radio Group A group devoted to Hacker radios with a huge archive of information and expert members. You have to join to access it but they are a welcoming bunch. Membership also entitles you to drink in 'Ye Olde Tin Whisker,' the virtual pub, where you can spend the evenings chatting about things non-Hacker, or even radio, related.
There are plenty of places that sell this information, often at stupid prices. The best of these is http://www.service-data.com/where a single top-quality download costs a mere two quid or, if you're really keen, you can buy an entire library on a DVD-ROM.