Charcoal is great for absorbing toxins and poisons that build up in the soil, years ago it was given orally in cases of over dose! but it doesnt last forever, dont get me wrong, its great to use, its the same stuff you use in aquarium filters, but remember that it has to be changed, also, I think your book is a bit out of date =( read through the post in this and other forums and you will see how far the simple concept of "gardening in glass containers" has come! People have little slices of rain forrest in there "glass containers!" Please feel free to drop in and ask questions, not to much activity lately, but a couple of us drop in from time to time =)

You can get horticultural charcoal in bags at most nurseries.