Hey there, I’m pretty new to trying to learn Sindarin and was just wondering if someone could tell me how accurate/ up to date David Salo’s book ‘A Gateway to Sindarin’ is. I am aware that it is probably not the most updated resource out there, but how behind is it? Thanks in advance for the info!
In addition to being slightly outdated, it's highly technical and not at all aimed at a beginner. More importantly, it tends not to make clear distinctions between Tolkien's Sindarin and Salo's brand of Neo-Sindarin. Here's an in-depth review noting these and other points of criticism.
The learning resources available today and most up-to-date with our current understanding of the language are Fiona Jallings's A Fan's Guide to Neo-Sindarin and these abbreviated notes of the same.