Have a look at Model Horse Tack School on YouTube! I only recently found out about it, but the tutorials look like great fun. You're able to purchase the patterns she uses for a small fee (they can be scaled to Julip-size). The tutorials are filmed in real time it seems, so it sounds like a great guide!
Alternatively, and if you are prepared to buy a download on the internet, you can get Susannah Bensema Young's guide to making tack for model horses for $11.99. Patterns for western saddles and harnesses, again all trad scale but I just downscale the patterns.
That video tutorial is 4 HOURS!! Well that's my evening entertainment sorted then! hehe I've also found Rachel Fail's channel which looks to have some nice content. Just need to be careful not to get distracted by cat videos.
Thanks for the link to that book Capriole, it actually looks very good value for money considering how much it covers. I'm sure I saw it for sale a while back but just as a hard copy and a lot more expensive. Just need to find a way to sneak it past the printer police at work...
Rio Rondo also does tack kits! I haven't tried out their Classic scaled ones but I suspect they'll be a tad big for Julips, though they do come with patterns and leather, so you could easily scale them down!