Even though Gardner's book is fairly formidable (445 pages; 1.7 pounds), do yourself a favor and read it before you buy and read the Urantia Book (2,097 pages; 4.3 pounds, per Gardner). By doing so, you will hopefully save the cost of buying, the time spent reading, and, most importantly, avoid a possible commitment to the Urantia Book.
I first ran across the Urantia Book (UB) in 1973, bought it and spent months of careful reading before finishing it in early 1974. At that time I got rid of the UB because I felt that, although unconventional, it was essentially Christian and capable of only producing yet another Christian sect, and was therefore limited in perspective and usefulness. The racial and religious prejudices, spread throughout the book but concentrated especially in the Jesus papers, were obvious, and likely not the product of those with a comprehensive view of the world, as assumed by the UB's purported cosmic authors. Because of the UB's complexity and obviously Christian focus, I concluded at the time that the source must be, say, a very cynical Christian mathematician.
Although I have not paid much attention to the UB for the last 30 years, I was very interested to recently run across Gardner's book in the library and to find that my initial reaction to the UB had some elements of truth. From Gardner's book, I learned that the authors of the UB, or at least its editors/compilers, were from a strong Christian (Seventh Day Adventist) background. Also, to date, approximately 50% of the UB has been shown to be directly copied or summarized from a variety of early 20th century religious, scientific, sociological, and historical publications that were available to UB authors prior to its publication in 1955. Of course this fact runs counter to the extraterrestrial origin claimed by the UB's naturally very secretive human sources. Whether their motivation was to create a special aura around the book, or to shield themselves from public accountability, in the short term it is clear that the miraculous origin gathered more attention than would have the open acknowledgement of its copyrighted human origin. Who would have given any attention to this eclectic mishmash if the sources had been duly cited? The fact that much of the material was illegally plagiarized is not in doubt, and is clearly presented by Gardner. This is not just his opinion, as stated by some UB devotees, because Gardner's book contains multiple instances of, and references to, identical passages copied word for word from identified, copyrighted sources into the UB. This activity has been shown, to date, to account for approximately half the UB; perhaps the remaining 50% of the UB is also plagiarized, or portions of it may have been authored by its editors/compilers. The complexity of some of the UB can be attributed in part to the complexity of some of its sources; at least one of the sources noted was a physics publication by an acknowledged expert of the day. Some of the testable scientific assertions presented in the UB were generally accepted in the early 1900's, but have since been discredited. However, to non-scientists unaware of the plagiarized sources, the complexity of the UB can be dazzling. This is understandable, because even though the accuracy of science increases over time, the complexity of any particular scientific theory is more likely a function of the individual author than its era.
If one wishes to be dazzled by complexity, why not simply pick up any college textbook of higher mathematics, particle physics, or astronomy and attempt to read that? If one doesn't understand it, or is amazed, confused, or dazzled by it, does that make the source extraterrestrial? No, because the authors don't claim such a source. Instead, they rely on their own effort, expertise, and availability as well as the honesty and testability of their work. The UB authors eliminate these sources of true credibility, and rely instead on the dazzle factor of the UB, as well as the significant credulity factor expected to be supplied by disenfranchised Christian readers, who are eager to see Jesus again at the top, even if in an unusual way.
As any potential readers of the UB have their resources, their time, and perhaps their very futures to consider, I think a skeptical attitude should definitely be taken before reading the UB. Fortunately, Gardner's book now makes the information available to apply this reasonable skepticism, and perhaps prevent some gullible individuals from being conned by undeserving charlatans.
If you have already read the UB, reading Gardner's book may help you achieve a new perspective. However, as Gardner notes in his book, in many instances this does not occur. It seems that the weaknesses of the UB, resulting from its particular human sources, tend to fit those of its devotees, resulting in a kind of symbiotic fit. To me this seems unfortunate, but not surprising. As these individuals are adults, they are obviously free to choose what they wish, and are ultimately responsible for themselves.