Well, I think in part this answer will depend on your definition of "source," or put another way what the purpose of the book is. If you mean a source suitable for use in academic writing then no, it's not, because it's a textbook. Textbooks are tertiary sources, which means they are generally compendia of secondary sources rather than original research. They can still be good for learning, but not for research.

The author of this book, Alan Farmer, appears not to be a trained professional historian, though he's written many textbooks and is/was a "teacher trainer," which I guess is a British thing I've never heard of.

I can't find too much information on the textbook itself, and the college library I'm at right now unfortunately doesn't own a copy. I suspect it's fine. Seems to be heavily weighted towards political history, which is not ideal but also not terribly unusual. Your experience with it will be entirely based on what your instructor does with it, I suspect.