I do not have the book you refer to, but I glanced at some of the pages using the "look inside" feature on Amazon. It appears to be a second edition (originally from 1999) and from what I can tell, doesn't look very up-to-date in terms of values. I personally only use these types of resources as a guide for relative values, since values change so fast, but you would think a 2020 edition would be a little more up to date. Note again that this is just my first impression based on a few example pages.
Amazon states that this is the "only reference you'll ever need for determining value of vintage oil and gas memorabilia." Realistically, there's no way you can expect a 254 page book that covers signs, cans, globes etc to be the definitive source in this hobby. It basically provides a broad sampling of the various items available to collectors (but I'm sure there's lots of interesting info provided).
Regarding your specific example of the 18" diameter magnolia sign: I'm not an expert on these but I'm only aware of them being produced in 30" and 42" diameters. I can also tell you that all of the 18" versions I came across being sold on ebay are garbage. It's also worth noting that many of the larger versions on ebay are repops as well.
