The Treasure of Alpheus Winterborn is notable mainly/only for being the only Bellairs book with no supernatural elements, and it also introduced Anthony Monday and his set of characters, whom Bellairs kind of neglected (The Mansion in the Mist, which I've just read -- and which is pretty good -- has them). The Anthony books included The Lamp from the Warlock's Tomb, one of his worst).
Bellairs was at his best with Johnny Dixon, which characters he obviously loved, and he wrote more Dixon books than any others. He made his debut in The Curse of the Blue Figurine; though the Johnny books in particular could be formulaic as the series went on, Figurine is fresh and one of his best. The follow-ups, The Spell of the Sorcerer's Skull and The Revenge of the Wizard's Ghost, are a high point in Bellairsiana; wisely, he never wrote a direct sequel again.
Avoid The Secret of the Underground Room and The Trolley to Yesterday. Especially with the latter, you'd think time travel plus the Dixon cast of characters plus the fall of Constantinople would be awesome. It's not.