No one needs another review of Anthony Doerr's novel. I enjoyed it. I did not hold unreasonably high expectations for it, and I was rewarded with an enjoyable time with a book. (That is not damning with faint praise, because an enjoyable time cannot always or even usually be found.)
In Cloud Cuckoo Land, the reader has to expend considerable energy and attention shifting from one focus to another. The action of the book happens in three general time periods, each with characters that arise from distinct circumstances. How they will intersect with each other, much less with consequences of characters' actions in one or both of the other time periods, lends the book a puzzle-like quality. Much less the fragments of an ancient text.
The everlasting problem of the mystery genre --whether historical fiction, mysteries of crime and redemption, spying and espionage, or fantasy fiction--is that the reader's attention must be divided between trying to figure who is doing, has done, or will do what, and who those characters really are. The truly best puzzle-books (I'm thinking of P. D. James' Adam Dalgleish novels, for example, or Robertson Davies' Cornish series) build the puzzles through the characters. Their actions reveal their hidden truths --and those of others, usually. Often the character who "did it" (and what is "it"?) appears in the five five or ten pages, but the layers of that character do not become apparent until much later. The reader has to attend carefully to character, because only through characterization will the key be found.
Anthony Doerr's characters were enjoyable but never more than two-dimensional. With one exception: Zeno Nenis, who provides a linkage between ancient text, contemporary libraries and children, and a future dystopia. His sexuality, confusions, and quixotic labors of translation provide insight for the reader into the deeper dimensions of literacy.
"Sometimes the things we think are lost are only hidden, waiting to be rediscovered." True of ancient texts, of human desires, of all our fears for the environmental climate cataclysm facing us. More attention to that sentence, in the lives of the characters, and this book would have moved from enjoyable and very good to truly great.