Any suggestions for preparation
Welcome to Juniper certifications. As Moses noted above, you want to start by reviewing the layout of your chosen track. Each Junos track consists of four exams and they are each a prerequisite for the next. So to earn a JNCIE-ENT you will take three written exams in this order:
JNCIA-JUNOS
JNCIS-ENT
JNCIP-ENT
For the first two exams there are study materials posted on the Juniper Fast Track Portal. Go here and create an account. You can download and follow the list of material to prepare for the JNCIA-JUNOS and the JNCIS-ENT exam.
The portal also has a sample test which will earn you a discount voucher to take these two exams.
https://learningportal.juniper.net/juniper/user_fasttrack_home.aspx
For the JNCIP-ENT exam you will need to review the exam objectives on the exam page and study independently. This does list the Juniper courses that support the exams if you want training. This training material is also available for self study purchase.
There are two books from O'Reilly press that support the ENT series. Junos Enterprise Routing and Junos Enterprise Switching.
As you encounter specific questions during your studies, post them in the Routing or switching forums on this site.
Best wishes on your studies.