Try Bryophytes of the Pacific Northwest. If Pojar and Mackinnon don't list R. elongatum at all it might be worth also checking Niphotrichum elongatum. There is some debate over whether to split the genus.When I had time I checked my Pojar & Mackinnon and they only list Racomitrium aciculare in this area, but I can't see much difference between it and the elongatum.
https://www.bryophytes.co.uk/ was the first hit I got, Bryophytes of the British Isles the second, The British Bryological Society the fifth, and https://www.naturebureau.co.uk/bryophytes a little further down. At a glance, these all look fine for a start. I also see a Facebook group which appears active.Can anyone recommend a good source to learn more about mosses, esp as found in the UK?