Yes, the Juniper courses have always been the authoritative source for certification material as outlined on the certification pages.
But these are more expensive than individuals can afford and we don't all work for companies that will pay for this official training with multiple days out of the office.
Juniper with the old Fast Track program kindly provided 3 free pdfs as a study guide for the jncis-sp exam. These were a great boon for the individual learner and I was sorry to see the program end and the books drop off the web.
Other programs have options to purchase study guide books similar to this set of 3, usually for less than $100 per exam. This is an affordable option then for individual learners. And can sometimes be expensed and paid for by companies for employees too.
I hope that Juniper eventually gets official study guides put together for the various exams. While in person training is ideal, this is not in the cards for most of us. Study guides are a great convenience to organize the material needed per exam.