The manual claims to have information on building trailers, but this is barely a page and doesn't amount to much more than "buy a kit" or "use a caravan chassis". There is minimal technical information in that respect, and only a brief overview on things like suspension types.
The manual covers the regulations regarding trailers and towing well though, and is worth getting on that basis, but as a resource for planning a trailer build/refurb it's not much use.
The focus certainly seems to be on an overview of what trailers are for and what you should look for when buying.
In summary, plus sides:
+Regulations are clear and detailed
+Good guide on choosing and buying trailers
+Good information on maintenance (wheels, brakes, etc, but not suspension).
+Good guide on towing and the required licenses / courses
negatives:
-Almost no information to help you design, build, refurbish a trailer or replace components.
-Lots of images of minimal relevance, missing some images that would be useful (what various components look like, what to look out for).