Keep Circulating the Tapes: These novels were last printed in the 1980s, to say nothing of some of the books Gaskell cites as sources. H.S. Bellamy's work in particular is nearly impossible to find.
The bibliography in the back of The Serpent and The Dragon consists mainly of speculations into prehistory, though a few sources differ from this topic. Of the books cited, the book of Genesis, James Churchward's The Lost Continent of Mu, Robert Graves's The White Goddess, W. Scott-Elliott's The Story of Atlantis and the Lost Lemuria, and Harold T. Wilkins's Mysteries of Ancient South America and Secret Cities of Old South America are still readily available. Most of the other books are out of print, though some of them can still found at used bookstores of both the physical and online variety. With the works of H.S. Bellamy, Phyllis Cradock, and Dmitri Merezhovsky, you have a problem.
Science Marches On: One of the most overt instances of the trope in action. Both The Serpent and The Dragon conclude with bibliographies of mostly discredited reference books. Most famous among these are Robert Graves's The White Goddess and James Churchward's The Lost Continent of Mu. note The Margaret Murray hypothesis, on which this series depends to a certain extent, wasn't discredited until the 1970s, but Some Summer Lands was published in 1977, thus pushing it into MST3K Mantra territory. Then again, Gaskell didn't really have a choice by that point.