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.
The Valve Page A 'Virtual Museum' featuring a large collection of valve radios, televisions and test gear. Includes some service data.
The Valve Museum A huge amount of information about valves made feely available.
ATV Aerials All you could possibly want to know about aerials; TV, radio and DAB. As well as being useful for technical information it's written in a very entertaining style.
Past Times Radio The site of a professional repairer/seller of vintage radios. He also sells old radios in 'as found' condition for people to repair themselves.
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.