One of the best study guides around: VCIX-NV Study Guide | Lostdomain.org
It hasn't been updated for Cross-vCenter NSX yet but those operations are very similar to a singular environment, so studying up for the difference is not too hard.
Another good source of information in general but not directly mapped to the exam objectives (but it does cover Cross-vCenter NSX): https://www.packtpub.com/virtualization-and-cloud/learning-vmware-nsx
