Hi Amber,
Here is a copy and pasted para. from a PM I sent someone recently. I haven't reviewed it but I can always come back later and edit if I need to
'have just had a rummage through the books I used a lot to prepare for Grade 6. I'm not sure how far you have got with basic harmony but I would suggest doing lots of work getting as full a grasp as possible before even looking at the Big 6 AB workbook. The workbook doesn't really give you much help on how to do the actual harmony but focuses more on how to do their exercises for the exam. I used a very old book (which got me through my ALCM dip theory in the 80s) called 'First year Harmony' by William Lovelock. It might be available second hand somewhere. It's a bit dry but it does explain. From the Butterworth book I would suggest chapters 1-5, 7, the basics in ch. 9, 10, 11 and maybe 12. The rest is aimed more at Grade 8 I think. In Koh I kept referring to pages 13,14 constantly.
For playing at the keyboard the Dorothy Pilling 'Harmonization of Melodies at the Keyboard' Book 1 is very good as it makes sure all the shapes are understood for root and inversion chords. I also like the 'Harmony is Fun' books. They don't go very deep but they are quite helpful with a 'family' analogy for the relationships between important chords and when they are used. You could also analyse the harmony in your pieces or hymn books are excellent for this'.
HTH
I know the book goes beyond Grade 8, so my guess would be it's not something to work through from start to finish. So I wondered which parts are needed for Grade 6? Also, if I were to take Grade 6, what else would I need to cover? I do have the MTiP Grade 6 book, but to be honest, at the moment I can't understand enough of what's in it to work out how it compares to Harmony in Practice.