Both are basically books for experts, and not technically in the same niche although they mainly focus on security. It depends on how much you want to learn and what topics you intend to master.

Security Design Consulting: The Business of Security System Design obviously tackles the basics of being a security design consultant, the process of security design and its difference with security design management. It offers a comprehensive scope regarding the responsibilities of such profession plus some tips on how to become successful in the business.

Vulnerability Assessment of Physical Protection Systems on the other hand is a more complicated source if you want to master vulnerability assessment (the safety, security and quality) of PPS, whether these are commercial, residential or of national importance. Simply speaking, this book by Mary Lynn Garcia is more of a technical reading material, there are charts and equations that would make your head spin especially if you don't have any background about the topic.

There are good reviews out there you might want to read before buying. Amazon's listed reviews on their page about the product (especially for the Security Design Consulting) but if you are no pro, or if you are just, let's say, browsing through a list of books, then just focus on the simpler ones. A Book Review On Security Design Consulting: The Business of Security System Design is a simple and understandable example of a review to give you a clue about the book.

Both of these books are really informative but I suggest you focus more on the one's closer to the topic you are looking for. Goodluck!