When it comes to books by Ellery Queen, there's a lot of disagreement among traditional mystery readers about which of those books are their own personal favorites.
Frederick Dannay and Manfred Lee, the two cousins who wrote as "Ellery Queen" went through at least three more-or-less distinct periods in their writing. Personally, I prefer their early novels, the first nine books about Ellery Queen - for newcomers to EQ, the detective character has the same name as the pseudonym used by the authors. Those early books all have titles that include a nationality, a noun and the word "mystery." The Egyptian Cross Mystery, the book we are discussing here this week, is one of those books. My own favorites among them would be The Siamese Twin Mystery and The Greek Coffin Mystery, but they're all pretty good.
Some readers prefer books from Queen's later periods, especially those set in and around the fictional town of Wrightsville - books, they argue, that are more mature than earlier Queen novels and have less of the puzzle element, while the books' characters are better developed and the overall tone is darker. Personally, I prefer the earlier ones, where there's more of a direct challenge to the reader to uncover the clues and solve the mystery before everything is revealed at the end of the book. But it's purely a personal choice. Among the later books, I'm very fond of Cat of Many Tails, their venture into the world of a serial killer.
If you enjoy the Ellery Queen books, I think you might enjoy two non-fiction books in particular that will give you more insight into Dannay and Lee and ow they worked and wrote. They divided the labor very strictly: Dannay came up with the plots and provided detailed outlines; Lee took the outlines and turned them into finished novels. They fought often, and bitterly, over their work - but they wound up writing some incredible books, and their influence on the American traditional detective story cannot be overstated. Francis M. Nevins' Ellery Queen: The Art of Detection is a first-rate critical biography by someone who knew the cousins very well. And Blood Relations: The Selected Letters of Ellery Queen, 1947-1950, edited by Joseph Goodrich, is a fascinating book of letters exchanged between Dannay and Lee when they were working on three books between 1947 and 1950 - not the "nations" books, but among their best.