I heard alot of good things about this book as well I will have to pick up a copy
The Gilly is one of my all time favorite fishing books. Each chapter is from a different author and is quite an enjoyable read. However it was originally written in the early 80s, so the comments about parts regarding gear and patterns being quite dated is very true. Don't buy it expecting cutting edge info. But entomology and basic techniques don't really change and I still re-read the odd chapter just for fun.
while they are all good books, some of the information on techniques and patterns in The Gilley and similar books like the 2 Jack Shaw books (Tying flies for Trophy Trout & Fly Fish the Trout Lakes). Morris and Chan on Fly fishing Trout Lakes. Steve Raymond's Kamloops, may be a little dated these days. Trout School by Mark Hume and Mo Bradley published in 2019 would be worth checking out.
Trout School is similar to the Gilly in that's is written in a similar 'anthology' style to the Gilly. Kind of half fishing info, and half fish tales from the golden age of BC interior trout fishing.
'Morris & Chan on Fly Fishing Trout Lakes' is a slightly more technical treatise on fly fishing BC stillwaters. It doesn't include the fish tales and anecdotes, so it's more of a reference guide and is more informative rather than fun, but ends up being quite a bit more comprehensive. Originally written in the early 2000's I think, but was recently updated and re-printed just a few years ago. At least it contains more up to date patterns that utilize modern tying materials.