I think if your going to write a book and only repeat what other people have already done, you have no business writing books.
Dont believe me, open a book by Hibbeler and then get Beer-Johnston. They are the same book. How is that not plagiarism?
The whole point of writing (such) a book is to have your name on a cover and have it sold. That's basically the answer to your question.
If I have to run to another book to understand whats going on, there is a *problem* with the book Im reading. The point of a book is to get a point across. If the author cant do that, trouble.
Now, that's a relative point. Someone else would perhaps understand what's going on where you wouldn't, and vice versa.