This is such a great series, and I am so excited for the 4th installment. Full of action, diverse and compelling characters, humor, history fun facts - seriously, I always learn a lot from these books (my favorite tidbit from this one was how turkeys got their name) I love when you can tell that an author does their research.
I will say that the romance could be more compelling, Phoebe gives me Mary Sue vibes and is just generally uncompelling as a character. Daniel, on the other hand, is great - a comic relief character who is actually quite intuitive, he's reluctantly brave, an amazing friend to Alexander and Phoebe when she lets him, and he takes his licks with little complaint. I love the relationship between him and Alexander who never treats him like an idiot and instead praises his talents as the strengths that they are.
I think Alexander is going on my list of favorite characters of all time. He's very well written with flaws that are realistic for a teenager (even one who is centuries old). Of course he's also very smart, not just as a benefit of his unnatural age but because he takes the time to learn and is curious, putting himself in positions to learn what he's interested about. Beyond that he's a compassionate character with the best sense of humor.