Nate the Great and the Big Sniff (Nate the Great #18) by Marjorie Weinman Sharmat, Mitchell Sharmat, Martha Weston (Illustrator)

Nate, the great detective, depends on his dog, Sludge, to help him solve all of his cases. But Sludge can’t help him this time — Sludge is lost! Lost inside a big department store on a rainy day. Nate frantically searches for his best friend and trusted helper. Salespeople have spotted a wet, slippery, sloppy dog running about. Is Sludge hiding from them? Nate looks high and low, and in places he’d rather not look at all. But every clue leads to a dead end... until Nate realizes that Sludge can help him with this case!