Hi Richard,
Well you got the wrong book and a questionable grill.
Some NB products have the exhaust entry at the top of the cooking chamber rather than at the meat grate level. This means that the heat goes straight out the top. Usually that is where they also mount their thermometer where the temperature may be 100° hotter than at the meat level. Another problem with NBs is air leaks. You will have to stop up all the cracks and crevices in order to control the temp.
Now, if you had bought "The Great American Barbecue & Grilling Manual" you would have turned to the chapters on "All About Grills" and learned about grill function, design and operation, including how to test your thermometer's accuracy.
Click into "According to Smoky" at www.barbecuen.com and read the "Glossary," "Burning Wood & Blowing Smoke" and "Heat" for some basic information.
Then I highly recommend that you click into the Barbecue Store at www.barbecuen.com and pick up a copy of the book and a digital thermometer with a remote probe. Both will make your life much more pleasant. With the thermometer, you start with the probe laying on top of your meat and you get an accurate reading of the temperature the meat is getting. When it gets near time for it to be done, insert the probe into the meat and you can tell exactly what the internal temperature is. No more guess work. The book will entertain you while it teaches you how to cook with ease and certainty.
Have fun,
Smoky