A more truly horrid book I don't think I could dream up in a million years. 
 Not only is "The Mark of Athena" a terrible piece of Children's Literature, it's just a lousy story in general with very little (if anything) redeemable about it. 
 Now before the Rick Riordan fans come at me with pitchforks, please allow me to explain. 
 I used to love Mr. Riordan's books very much. I enjoyed the five original Percy Jackson books so much I almost felt they were on par with series such as "The Chronicles of Prydain" and "The Chronicles of Narnia". I even enjoyed the first two Heroes of Olympus books overall, particularly "Son of Neptune", which was a surprisingly strong and engaging novel. 
 I'd looked forward to "Mark of Athena", even rushing out and buying a copy at the first chance. 
 Sadly, Mark of Athena was basically a giant wad of spit hurled at all that was loveable about the original series. 
 First, the novel uses the new POV (from Annabeth for the first time) to try and take back facts confirmed in previous books, under the crappy excuse that "Oh we just didn't KNOW it was this way, because we've never been inside her head before". I'm sorry, but that's bull. 
 The original books make it clear that Annabeth's crush, as a twelve year old, was still on Luke, her affection for him constantly a factor in her desire to redeem him. However, her character development had grown so that, by the end of the fifth book when he asked if she loved him, she was able to say she only loved him as a brother, as her feelings had romantically shifted to Percy after getting to know him over four or five years. Makes perfect sense. But in Mark of Athena, Annabeth suddenly proclaims she had a crush on Percy during their first year together (when they were 12). Not only does this mess up the original, but even The Lost Hero states that Annabeth loved Luke first. 
 The great thing about Annabeth and Percy's relationship was how it had grown from a reluctant friendship into a mutual romance based on their personalities. Mark of Athena makes it seem as though they've just been in love with each other's looks from day one. 
 Provided it's done well (aka, Romeo and Juliet, and some aspects of the Zoya/Georgy pairing in the novel "The House of Special Purpose"), I don't have a real problem with "love at first sight" being used as a literary device. But Annabeth and Percy were already previously established as NOT being a love at first sight couple, and therein lies my problem with the jarring change. If Riordan WANTED a love at first sight story, he should have invented new characters to do so with, not changed previous characters. 
 Second, the novel takes back the message of the original series regarding bullying, and that REALLY bothers me. In the Percy Jackson books, bullying was negative. Nancy's treatment of Grover in book one almost made Percy go postal. In the fourth book, Percy has to take a step back and look at himself objectively when he realizes he may in fact be bullying someone into helping him accomplish his goal. Luke is shown to have gone from a kind, caring figure under stress into a bullying villain after betraying his friends. Bullying was, in short, a big no-no. And what could be a better message for kids than that? 
 In Mark of Athena, bullying is shown as...sort of okay...? Or a gray area? Leo bullies Echo and it's shown as him just "teasing her for fun". There's a feeling of eye-rolling going on when a passing character proclaims that "scrolls" are the only "real" books in a painfully obvious jab at people who dislike kindles. So, what, is Riordan saying it's okay to laugh at someone, or just act like they don't know what they're missing, if they don't like electronic style media? That's kind of a crappy thing to teach children... I'm sorry, but if I had a child and I found out they belittled an adult (or even snickered behind their back) for preferring literal books to ebooks, I'd take away their kindle for a MONTH and they'd have to go up to the person and APOLOGIZE. 
 Thirdly, and perhaps most heartbreaking of all, Annabeth's troubles with her parents are "explained away". In the original books, we see a young girl grow into a young woman and have ups and downs in a blended family. All of this is thrown out the window with a short story told from Annabeth's POV, highlighting that her stepmom simply didn't know Annabeth was having bad dreams about spiders, it was all a misunderstanding, blah blah blah... The heck??? Now Riordan is trying to say the parent is always right or that the child is at fault for not communicating better? What a load of bull! Yes, a loving parent CAN make mistakes, and children SHOULD communicate what's troubling them, but there ARE some parents who simply make bad choices or just refuse to understand their child's needs. The original books, with the ambiguous portrayal of Annabeth's parents and the villainous portrayal of Gabe, I feel, was much more realistic to children who may have serious family problems that cannot be resolved with "more attention" or a "family meeting". It is a COMFORT to such children reading such things, seeing they're not alone. To snatch that away in a sequel series is low. 
 It is also annoying that characters in this book never seem to give up anything, or lose anything irreplaceable, in their pursuits. It also makes the story laughable and, to be honest, painfully boring. Aside from the points that bothered me and made me want to hurl the book across the room, very little of the story stuck in my head after I finished it. It was dull, unflattering, and possibly unedited (there are some very painful typos in this book, Riordan, please get a better editor!). 
 For anyone looking for similar, better books, I'd recommend "Voyage of the Dawn Treader" by C.S. Lewis for younger kids, and if you're an older teen disappointed by Mark of Athena, try City of Bones By Cassandra Clare. I realize City of Bones has gotten some bad press, but it's actually a pretty dang good book and a heck of a lot easier to stomach than Riordan's recent works.