How do I get access to the above book written by WP vd Merwe. I am married to a descendant of FJ Strauss.
Welcome to the "Community.FamilySearch" Forum.
I am just another 'lowly User/Patron ...
I just checked the 'FamilySearch' "Catalogue".
I believe that this is the "Book" that you are referring to:
Assuming such is the case ...
As the "Book", may not have been "Digitsied"; as, there is NO "DGS" number; or, reference to "Digitisation" ...
And, as the "Book's" availability, is in "Off-Site Storage", through the "Family History Library" (FHL), in "Temple Square", in Salt Lake City, Utah, United States of America ...
There are a couple of options:
 In Person
IF, you can (ie. have the opportunity to) visit the the "Family History Library" (FHL), Salt Lake City, Utah, USA; THEN, prior to your intended visit, to the FHL, you will NEED to make a formal "Request", for that "Book", to be "Retrieved", from "Off-Site Storage", to coincide EXACTLY with your intended visit to the FHL.
Here is an Article in the 'FamilySearch' "Wiki":
With the following link:
To order an Off-site storage book into the Family History Library:
[ 2 ] Externally / Remotely [ ie. NOT in Person - if you cannot visit ]
IF, you CANNOT (ie. DO NOT have the opportunity to) visit the the "Family History Library" (FHL), Salt Lake City, Utah, USA, like many; THEN, you may like to avail yourself, of either, or both, of the following:
There are two possibilities here:
(a) FREE Virtual Genealogy Consultations
.... Virtual Genealogy Consultation FAQs
(b) FREE Family History Library Record Lookup Service
.... 'FamilySearch' "Blog"
.... New Library Lookup Service—For When You Can’t Visit the Family History Library
.... Family History Library
.... Family History Library Record Lookup Service
There are also THREE (x3) copies, of that "Book", in Universities, in South Africa, according to "WorldCat":
(1) North West University Library
..... Ferdinand Postma Library
..... Potchefstroom, 2531 South Africa
(2) Stellenbosch University Library
..... Stellenbosch, 7602 South Africa
(3) University of Pretoria
..... Pretoria, Gauteng, 0002 South Africa
I hope this helps, somewhat.
A small addition to what @Brett . has proffered above:
perhaps some other books on Strauss will be more easily accessible?
I hope this helps.