In reply to BigLob:
"Mountaineering in Slovinia" by Tine Mihelič is quite a good "inspirational" guide which contains via ferratas routes, but it's quite difficult to glean exactly how difficult a particular line will be.
https://sidarta.si/en/product/mountaineering-in-slovenia-en/
The best pure VK guide I have seen is only available in german as far as I know. I've found it pretty easy to follow generally though. I'm pretty sure I have an older version of this:
https://www.rother.de/en/klettersteige-julische-alpen-1.html
In addition I have a mini guidebook that purely focuses on Triglav and another for the Kranjska Gora area. Can't find them at the moment though.
There is a good bookshop in the alpine museum in Mojstrana that you should definitely visit. Last couple of times I have been there it has had a good selection of maps and english language books that are hard to find in the UK.