There are a couple of unofficial study guides for VCAP-DCV - mostly for old versions (5.5 or 6.0), but they are still great for preparation.
They helped me to pass the exam in this summer :-)
Books I read to prepare myself for VCAP - Design DCV :
1. VMware vSphere 6.x Datacenter Design Cookbook by Hersey Cartwright
2. VMware vSphere 6.5 Cookbook by Abhilash G B and Cedric Rajendran
3. VMware vSphere 6.5 Host Resources Deep Dive by Frank Denneman and Niels Hagoort
I read book number one a lot, more detail, cover to cover. The rest is also very good resource.
Beside, I also read blogs from VMware enthusiast sharing their experience about this exam. You can find easily this on the internet.
What I have to master for this certification? You must be able to identify Risks, Assumptions, Constraints, and Requirement. Also to differ between non-functional and functional requirements. Must know well Availability, Performance, Management, Recoverability, and Security (APMRS) components. What’s else? Upgrade process from 5.5 to 6.5 version, Content library, vCenter Linked Mode, VMware encryption, RPO, RTO, SRM, vSphere Replication. What’s else? Please refer to the official Exam Preparation Guide.
And if you have some budget for training, take 'VMware vSphere: Workshop Design' course.
I hope this info can help your preparation. Good luck.