For what these books are, they are fine. They are even a fun read. Just don't over-think them. Such as, with all the information about some obscure mythological characters, there was really no explanation as to how Atlas was released from holding up the sky. It's been about thirty years since I've read mythology, but I'm pretty sure the original Perseus turned Atlas to stone with Medusa's head and that gave us the Atlas mountain range.
The other thing that I'm finding happening too much is when Percy flashes back on previous conversations or memories. Flashing back is a useful devices, but not when it enters every other chapter.
Still, these are fun books to read.