Those two are probably the best, but there are also books on both companies:
"The Crystal Years: A tribute to the Skills and Artistry of Stevens & Williams Royal Brierley Crystal" by RS Williams-Thomas
"Art, Feat and Mystery : The Story of Thomas Webb & Sons, Glassmakers" HW Woodward
Of course there is also the recent book by Charles Hajdamach "20th Century British Glass".
Unfortuanatley, none of them give you the information that I have given you - such as that is :huh: All of them give clues to some characteristics, but much of what I say is gained through experience of handling items, combined with a modicum of research. Mostly, they seem to deal with the more up-market items by both companies, although Hajdamach does discuss some of the production wares, particularly from the post war era, with some emphasis toward cut glass (the work of David Hammond for Webb). Some of the things I have identified to do with S&W over the years are discussed in the text of Hajdamach, but sadly not illustrated.
I hope that may be of some help, Nigel