I'm not sure if you are aware, I found article CS200084 (see link below), and according to it, there is facility (conversion of documents to Prime format + VBS scripts to build linked HTML pages) to convert Mathcad 15 (and earlier) eBooks into HTML hyperlinked documents that automatically open in Mathcad Prime. The article describes the 8+ extension packs that came with various older versions of Mathcad (15, 14, 13, 12, etc.) that included "Topics in Electrical Engineering", "Finite Element Beginnings" (Febeg.hbk), "Building Thermal Analysis" (Thermal.hbk), "Building Structural Design" (Structur.hbk), "Roarks formulas for Stress and Strain" (Roarks.hbk), "Machine Design and Analysis" (Hicks1.hbk), "Electrical and Electronics Engineering" (Hicks2.hbk) and "Electrical Power Systems Engineering" (Power.hbk). It also says the scripts are easily adaptable to any custom eBooks. I haven't tried the process yet, so cannot comment on how successful or effective it is.
Although there is still the HUGE loss of the live math interface of eBooks and the ability to annotate them, and copy information from them directly into Mathcad, at least there is a simple way to access the information and preserve some of their value. Check out the link below for a Word doc explaining the process and a link to the required scripts. Meanwhile, I've attached a screenshot of the result (from the link below).screenshot.JPG

BTW, I voted for the eBook suggestion for Prime 5 (there were only 6 votes including mine
).
The link to all this information is here:
https://support.ptc.com/appserver/cs/view/solution.jsp?n=CS200084
David