If you are looking for restoration guides, Porter’s 1st edition was the earliest serious attempt but had a lot of mistakes, 2nd edition fewer, 3rd edition getting closer. Schmid has a few mistakes. Viart has more, but his pictures are really good. Even the factory Spare Parts Catalogue has quite a few, I think 62 last time I counted.
The difficulty with the 120 is there were so many changes throughout the production run, some well recorded by the factory, some poorly or incorrectly, some not at all. Many cars received modifications at the factory, or dealers, or speed shops, or local garages. The interest in and pursuit of original authenticity only began to gain momentum in the 90s with Porter. Completely untouched unrestored original cars are rare, and each only represents a brief period glimpse out of the six years production.
Another that wasn’t mentioned is Jaguar XK120 in Detail by AD Clausager, some production records including colors and shipping to dealers, and a lot of interesting history.
The JCNA Judging Guide benefits from one aspect neglected by the others, a principle particularly recognized in scientific research, that is peer review and revision before publication. I was on the review panel. It is not intended as a bolt & nut restoration guide, it is for judging the visible things, such as which cars should have chrome sidelights vs which cars had painted pods, which cars had black engine and luggage compartments vs body color, which had glass brake fluid reservoir vs steel, how to recognize an original color scheme and upholstery pattern vs a California custom job, even variations in the brass data plate and errors in reproductions and restampings.
Going back a few decades and covering more history and less about restoration, there is Jaguar XK by Chris Harvey, Jaguar XK Series by Jeremy Boyce, Jaguar XKs by Paul Skilleter.
Before the internet, it was often very difficult to find answers to the simplest things. I looked for 2 years to find an example of the early gearbox mounting shelf, 10 years to find the explanation of the junction box on my scuttle, 20 years to find a correct map drawer to copy (mine is the 2nd of 3 versions). One of the features of this forum from the beginning is that it has always been about accurate information. What you do with the information is entirely up to you.
Of course this level of detailed research isn’t everybody’s cup of tea.
Some people say just drive it and have fun with it, as the original owners did.
My brother the Porsche 928 owner looks at my book collection, shakes his head sadly and says, “You are turning into Corvette people, and the 356 people are closing in on you.”