Gaaahh.... I don't know how to review this. Perhaps I'll just blather and see what shakes out. 
 This was sort of like Miss Peregrine's Home for Peculiar Children in that the premise has all the ingredients of a 5-star read for me. A mysterious vertical bookstore and a likeable nerdy protagonist who stumbles onto a puzzle involving old books, a secret society, etc, etc. I was absolutely in love for the first 50 pages or so, too. And then... I don't know. 
 Maybe it was partly the same problem as with Miss Peregrine. The stakes never felt high. The relationships weren't emotionally deep. There wasn't much of an arc for the protagonist. The narrative went, "I'm a likeable resilient guy and here's a cool happened to me. I'm still likeable and resilient!" Okay. (I admit I didn't like the shift in tense partway through the book either.) 
 Some of the puzzle/mystery stuff seemed superficial to me, too. (view spoiler)[Even the climactic scene, where all the Google people had to do the codebreaking attempt in an orchestrated fashion, seemed contrived. It wasn't clear what they were actually doing. It was like, "We will do codebreaking to it!" with some strange imagery that left me unsatisfied. (hide spoiler)] I wasn't expecting a cryptography course, but what little he gave me didn't quite suspend my disbelief. 
 It is a fun read and I liked some parts of it ((view spoiler)[the amazing bookstore, the idea of immortality through the printed word, and the fact that the key was something related to an artifact related to physical printing (hide spoiler)]) and I can totally see how people fall under its spell. I just wanted more. (So greedy, I know.)