The central argument in this book is that huge Chinese fleets charted pretty much the whole world in 1421-3, and their maps guided the European explorers, from Columbus to Cook.
The most interesting and credible material in this book (p. 382-7) is for the most part identical word for word to a 1977 article in the Geographical Journal, Vol 143, No 3, p 451-9. Menzies does not credit the source, mind you. Read the original rather than Menzies corrupted version. You can find it on the web too. Search for Martellus world maps by Arthur Davies. It presents a convincing argument that the Columbus brothers faked a map to dupe the King and Queen of Spain into funding their project to sail west to Asia.
The rest of the book is nonsense. Menzies is not even consistent. For instance, he claims that the 1513 Piri Reis map shows the coast of Patagonia "with great accuracy," providing evidence that the Chinese had charted it before Magellan got there (p. 116). But on p. 377 he says (rightly) that the latitudes of the Orinoco and Amazon deltas on the 1513 Piri Reis map "are precisely correct," which places the Amazon delta on the coast he had identified as Patagonia! The two regions are on opposite ends of South America! Too make his case appear plausible, Menzies only shows a bit of the Piri Reis map, but when you see the whole map it becomes obvious he is placing Patagonia in the tropics! The whole map is in the colour plates between pages 200 and 201, but he does not refer to it.
Menzies reasoning and standards of proof are amazing. For instance he identifies the Satanazes Island on the 1424 Pizzigano map as the Caribbean island of Guadeloupe (p. 246-9), meaning again that the Chinese had charted it. Now Satanazes is rectangular whereas Guadeloupe looks a bit like a butterfly! To explain away the differences Menzies has his Chinese fleet sailing back and forth around one wing of the butterfly without ever catching sight of the other wing! But the rectangle still does not look like a wing (check out p 249). So Menzies has his Chinese hindered by darkness at night or blinding sunlight during the day or some other lame excuse, and in the end he has the nerve to assert that the island was charted accurately! And he does not tell the reader at this stage that he is shrinking Satanazes by a factor of seven, turning it upside down, and dragging it some 4,500 km across the Atlantic, from a place some 1,500 km northwest of Portugal on the Pizzigano map to the Caribbean where Guadeloupe is! I'm not kidding. As with Patagonia, he shows a bit of the map only, so you don't see where or how big Satanazes is. You can find the whole map in the plates between pages 296 and 297, but again he does not refer to it. You can get the map on the web too.
These are not isolated examples. Throughout the whole book Menzies misrepresents sources and facts, draws illogical conclusions from doubtful evidence, and makes bare assertions based on no evidence. If you want more examples look at an excellent online review, "Gavin's Fantasy Land" by Bill Hartz.