There are alot of books on the market about improvising, composing and keyboard harmony, but is there a book that anyone can particularly recommend to help GCSE and A level music technology students learn about chords and improvising?

There are 2 books by Laura Campbell, 'Sketching at the Keyboard' and 'Sketches for Improvisation'. Or if you want a more traditional approach to keyboard harmony there's a book by Dorothy Pilling 'Harmonization of Melodies at the Keyboard'. Not sure if this is still published, and not sure if any of these would do.......they are all practical books. If it's theoretical there's the book by Anna Butterworth published by the ABRSM on harmony. Plus Trinity & Guildhall have some stuff on Improvisation and Extemporization as it's a part of their exams. One of the best harmony books out was Annie O Warburton but I don't think they are published any more and that was theoretical not practical at the keyboard. (I remember using that at O level!!!)