This really sounds like it's only saying "De en bog," is the text-to-speech bad, or do I just need to get used to the sound of the language?
Danes are really lazy, so in normal speaking we tend to pull some words togheter
Why is it "Det er en bog" instead of "Den er en bog"? If "bog" has a common gender and it's called "en bog", shouldn't it be "den" also?
I mentioned this in another comment, but here: "It [unknown item] is a book = DET (neutral form). Thus, " Det er en bog" might mean, I didn't know what was in my bag, but turns out "It is a book"
If you know the item of which you speak, you will likely use the appropriate gender.
It's a question of whether you're speaking of a generic it vs. this specific item
"Den er en bog" could be right, but then you're most likely to say something like "that one is a book" or "that is a book" instead of "it is a book". There are also more words which use "en" but have "det" (and vice versa) when using a specific term. I don't know the rule for it (or even if there exist a simple one) but I know that at least in Swedish it's very confusing for someone who isn't used to the language when to use "det" and when to use "den". Good luck!