When you get conflicting information regarding Internet protocols, you can always refer to the RFC and the author of the protocol. The Internet Protocol specifications are explained in detail in the Request For Comments (RFC) documents. These documents are stored at the Internet Engineering Task Force’s website (IETF.org).
In our case, the definitive answer is:
RFC 1945: “Hypertext Transfer Protocol HTTP/1.0”
RFC 2616: “Hypertext Transfer Protocol — HTTP/1.1”
I respectfully add that Mr. Jackson has a mistake in his book.
However, in the grand scheme of the Universe, it is simply referenced as HTTP and nobody but teachers, professors, students and IT geeks really care about the correct expansion of the acronym.