It was SO bad. The worst one overall.
So many inconsistencies: The first one is in the very first page! The description of the drinking of the serum, along with the appearance of the serum itself, isn't the same as in the book before that. It's like Baldacci didn't even read it!
He didn't get the characters voices right at ALL, especially Fiske, who wouldn't shut up about how strong he is for an old man. (Baldacci, an old man, must be projecting.) Then Amy sounded like such a lovesick airhead, which she isn't! She is, or at least used to be, an intelligent and strong willed girl. Then everyone else just sounded like a generic character talking. I wouldn't have been able to tell who said what without seeing the character's name written afterwards because that's just how lacking they were in personality!!!!
So many crappy disproved science fiction cliches. The part with the train sucking them into the tunnel is so obviously unscientifically correct (watch the mythbusters episode) that I could barely get through it!. For a series that is supposed to be educational and falls under the historical fiction genre much more than the scifi one, this was so disappointing. Baldacci should be ashamed to have ruined the credibility that the series had established thus far.
Now about the deaths. Evan was killed off just so that Jamy could continue. I HATE Amy now. She mindlessly cheated on her steady and amazingly supportive boyfriend even though she could have behaved like a true role model and waited to speak to Evan and at least break up with him before pursuing another relationship behind his back. He died fighting the fight that he insisted on being a part of just because he loved Amy THAT much and all she did was stab him in the back. Poor guy died without even finding out. The suspicions he had that were built upon in the previous books led up to nothing thanks to rushed and sloppy writing.
Don't even get me started on how bad the reveal of Damien and Damien in general was.
Honestly, I couldn't get through five pages at a time without having to put it down and take a breather because I was so appalled at how much of a mess it was Truly, there are fanfics better than this book out there. David Baldacci...idk how he writes his other books but if it's anything like he did with Day of Doom, then idk how on earth he's a bestseller. What a horrible, horrible conclusion! It even made me stray from wanting to continue with Unstoppable.
Also, if you know anything about proper english writing, you would notice how awkward and downright CRINGEY his sentence building is. He also starts senteces with "and" and "but" WAY too many times. It's okay if you do it a few times, but too many and you start to understand why it's not allowed when writing your english class essays.
P.S. Day of Doom completely disregarded the building darkness and trauma in Dan and made him a useless comic relief character (the jokes weren't even funny!). It did the same with Amy's depression, making her seem like a "wuss", which just hurt my heart so much as someone who suffers from depression.
I could honestly write an entire book on why Day of Doom is subjectively AND objectively the WORST novel, not just within the 39 clues series but overall, that I have ever had the displeasure to read.