Vintage-Radio.com
A great place to find guidance if you're just starting to get interested in old radios.

UK Vintage Radio Repair and Restoration Discussion Forum
The largest of the forums and the only one worth bothering with.  The best place for technical information and advice.

WorldRadioHistory.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 freely 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.

http://www.radio-workshop.co.uk/manuals.shtml 
This place offers a reasonable range of free downloads of service information for vintage radios

There are plenty of places that sell this information, often at stupid prices and some are outright cons.  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.