Opinions mine.
People are trying to do what used to take a room full of PhDs and grad students to accomplish.
This stuff is complicated.
These days everyone wants to policy route this traffic and that out a VPN or two on a set schedule while containing DNS leaks with pretty much no knowledge of simple things like you can't have the same subnet on two interfaces. Then when you tell them that can't be done they argue "but it worked on product X." They want a turnkey product but don't want to renumber. That and they have to have IPsec working to their workplace and the IT department there is of less-than-zero assistance.
Sort of akin to rebuilding an automobile transmission - but the same people who would never attempt that because they don't have the knowledge, experience, or tools, somehow think that building a complicated IP network that functions well is somehow within the grasp of anyone and everyone. I do not understand that phenomena.
They won't even pay $25 for the book.
It's pretty ludicrous.