I think this book does a good job of explaining the first 60-70% of what you have to know to be a competent network engineer on the Juniper SA gear. I recommend it to my L2 staff for just that. You have to admit it beats the manuals, which seem to assume you already know what you want to configure, and the only goal is to tell you on which screen to do it.
I'd agree that the book does not get to advanced topics. Personally, I think Juniper falls down here - these should be on the support site. There are a number of documents there on specific configuration or troubleshooting topics, but Juniper does not appear to be committed to making sure they are up-to-date. And there could be a lot more - just as you said, a tutorial on custom sign-in pages would be very welcome.
I describe the Juniper SA as the "Swiss army knife" of networking. It would be really useful to have documentation on what cool stuff you could do with each blade.
I now have this book and I think it's a great read.
Keep up the good work!