i've just been reading
Making Better Flies by Lou Tabory, which I got out of the library in Auckland.
It's an excellent book - rather than just patterns (though there is that stuff), there's excellent analysis fly design, movement, the latest materials, colours in the water versus out, etc.
The target fish species, and baitfish the flies imitate, are obviously US-centric, but the information would be transferable for our waters.
It was published this year, and I think that's important. It's up to date with synthetic materials which you can also find online. The problem with older books is that the materials used don't reflect what's available now. anyway, something to consider.
https://www.amazon.com/Making-Better-Flies-Develop-Patterns/dp/0811711382
Another thing i'd recommend is keeping an eye out for copies of the Aussie magazine Flylife. You should be able to find some second hand. Some great salt water patterns and advice in every issue, but it also has trout stuff.