Or alternatively have a look at Waterway Routes, https://www.waterwayroutes.co.uk , in PDF form so can be browsed on your device of choice and printed out already formatted for A4, can be bought as a download or a DVD by post (which also gives download link) so delivery can be as quick as your internet connection. They won't give the incidental detail or history like a Pearsons or Nicholsons, but do provide all the relevant stuff like moorings, services, water, marinas and what they provide.
As to your original point, i have a couple of Nicholsons guides saved in my Amazon basket for later (to replace our oldest editions at some point) and had seen a couple of weeks ago that they were suddenly unavailable, this would explain it. I looked at the publishers website, Collins, and they too show temporarily out of stock.
As they are printed in China this may explain it.