In mathematics as well as in programming, there are rigorous and less rigorous texts. Compare “Linear Algebra for Dummies” with “Principia Mathematica”, for example. I haven’t read the first or more than a page or so of the second, but I think you’ll find the latter more rigorous.

Similarly, in programming, “Teach Yourself PHP in 24 Hours” is wildly different from “the Art of Computer Programming” (a series of books that you may like reading)