Guinness book of records (covers nearly any happy to read 12 yr old so really safe bet)
Dangerous book for boys - Gonn and Hal Iggulden (great for imaginative lads)
The new way things work - David Macaulay (more entertainment than you might think courtesy of a woolly mammoth)
Underground - David Macaulay , if any interest in the world under the pavement. (Curious boys and geeks of all ages in 7th heaven!)
Pretty much anything by David Macaulay if an inquiring mind, stick with ones involving the happy mammoth if wanting to keep it lighter.
The Human body - Dorling Kindersly if reasonably mature 12 and interested in how it works. (Brilliant book fantastic cross section illustrations that every family should have. Best £10 investment ever! Does cover pregnancy, sexual organs and stds as well as other medical conditions, so not for everyone but I?d give it to every 12 year old! )
A Little illustrated History of the World: EH Gombrich, if any interest in man, civilisations, history (excellent adult historian but doesn't talk down, presented as a narrative of from stone age man to atomic bomb manufacturer, to a friend?s child.)
Usborne Illustrated Dictionary of Science if bright spark (highly practical good basic reference of all 3 sciences, will carry through into later years- slightly modern text book feel- boring if not interested in science)
The self marinating rabbit This book is surely just begging to be written! 