It's sometimes hard to realize that all tools and appliances began as gadgets: mortar and pestle, bow and arrow, toaster, vacuum cleaner, pencil sharpener, telephone. Even so, and despite the millions spent each year to research and develop a better mousetrap or what Leonard Silver of Hoffritz, Inc. calls FADs -- items used For A Day -- who would believe that some of these gizmos are going to catch on. Like the pushbutton salt and pepper set, or 'the Belly Beeper -- if your posture slumps it makes a reproving noise -- or even the battery-operated nail file. Grossinger, an admitted gadget freak, types them according to use: household/locks/safety, kitchen, bar, health and medical, grooming, office, travel, automobile and games and sports; she's more or less enthusiastic about all of them -- even the Poop-Scoop with which you may collect your dog's droppings from your neighbor's petunia patch. And if the book itself is something of a curio and a gimmick you can give it to Uncle Carl who's been collecting thingamajigs and whatchamacallits all his life. Illustrated and with a ""resource"" guide that tells you where to buy it -- many items are mail order only.