I'm not John, but I'll try to answer,
For me, the best "written" source is the Husqvarna Museum book - Originally "Husqvarna Hagelvapen 1877-1977" in Swedish and "Husqvarna Jaktvapen 1870-1977" as a swedish language reprint only - there is a very rare english version of the original book, but it's really unlikely you may find one.
There are some Internet sources, like Gunboards under the Swedish Civilian firearms (an US-Based International Collectors forum - of wich I am a longtime member), skydevaaben.com wich publishes a partial translation (with some minor errors) of the HVA book, some collectors / value books, but they usually are "in the grey" or full of errors, any HVA articles, in the Gun Disgest of the production period, former Tradewinds, Ruko, Ellewood Epps ads are also very helpful, anything else wich can help.
But, still the best source apart The Book, is first hand personal experience, from handlig / owning / dissassembling / repairing these rifles / shotguns. But still, after over 20 years of HVA love, I still learn and discover stuff on a regular basis.