This is an invaluable book for anyone interested in portrait painting. His methods are relatively simple and straightforward, that is not to say that it is easy as, with most things, it all takes practice but this book certainly starts you on the right steps.
Don't be put off by his description of how your studio should be set up, I'd need to win the lottery for a studio like his and also his list of equipment, particularly the brushes, will probably be outside the budget of most amateurs however his technique is very much achievable with much less equipment than he recommends.
All in all this book should be on the bookshelf of anyone interested in painting portraits.