Despite RnB's negative review of this manual, I went ahead and got one and it's fine for what I need. It's up to Haynes normal standard, i.e. some good clear parts, some confusing ones, some good photos and some unclear ones, but there is enough in here to get most jobs done without too much pain. I have used it to work on the cooling system successfully.
By the way, there are 2 pages on spark plug replacement, with photos, and the cabin air filter replacement is covered in the Subaru owners manual, so don't be put off. Subaru's are pretty bullet proof so you shouldn't need this too much, but it's always good to have a manual like this on the workshop shelf.