This is a review (of sorts) of a book that shall remain Nameless, broken down into all the things it did that you Really Shouldn't Do. Parts one, two and three are listed here.

Secrets are a vital part of story telling. Keeping - but foreshadowing - secrets are what generate the twist in a storyline, and give the reader that expected-but-unexpected ending they crave. But secrets have a fair number of unspoken rules, many of which are broken by writers-in-training.

Secrets must be secret

Duh. Though this isn't that common. This ties in strongly with foreshadowing-gone-wrong - telling your reader what that unexpected ending will be. Nameless actually did this a fair amount, usually in dialogue. It's usually a case of the author not giving the book to someone else to read, first. (Or maybe that someone else was just really dense. Or so bored they weren't paying attention.) But the deal is, if your story has a secret, you can't tell everybody about it. You can't even tell them there is a secret. That's rather the point.

The reader already knows - they know there's a secret. They know there's something they don't know. But while they're reading, they conveniently forget it. They immerse themselves in the story, willingly following the author's revealing of information. If you go blathering about secrets, you're inviting them to deconstruct the book and work out what that secret is long before you get them there - and secrets are rarely as entertaining or awesome when you figure them out before the buildup. If you have a secret, keep it secret.

Secrets must have a purpose

This is probably the most common mistake with secrets, although number three comes pretty close. If you're deliberately withholding information from the reader, it must have a point. A meaning. It must be intrinsically connected to the story so that when the information is revealed, all these other pieces fall into place, and the reader is amazed.

What you don't do is a secret just for the sake of keeping a secret. If the only purpose of the secret is to keep the reader mystified or intrigued until you reveal it, it's not helping your novel. Nameless broke this rule painfully, with two characters - an apparently good character, and apparently evil one, actually turning out to be the same character wearing a different hat, and they were both good.

And then nothing was ever done with that. We had a big reveal, the evil-version's misdeeds were swept away, and the book moved forward. It didn't care about what the evil version had inflicted upon the protagonist, or why, or even bother to address why the character was two characters in the first place. In short, the secret was there only to add the suspense of that apparent-adversary - who existed only to generate said suspense. 

Secrets must not blind your reader

This one can be difficult, especially when a secret is crucial and readily apparent if someone asks the right questions (which doesn't make the secret a bad thing - most murder mysteries are solved when someone works out what questions to ask). The secret must not rely on the lack of a simple description, or something that, had the reader physically been there, they would have noticed.

This has the most destructive effect on a reader's enjoyment. They're slammed out of the book. They've been told blatantly by the author that they're not actually there, that they can't see or hear or taste or touch anything. That they can't trust their senses, and they can't trust the author/narrator to tell them the truth. They can't trust anything in this book, because the author hid from them something that they feel they should have known.

In Nameless' 'meaningless' secret detailed above, our protagonist looked both characters squarely in the face, but the descriptions were completely different. We saw the story from the viewpoints of both versions of that dual character, but not a hint was made - when we were inside the character's mind - that they were one. As a reader, that second point was a clincher - that was something that I felt certain I should have 'seen', though her thoughts. The fact that I didn't meant I lost all trust with the narrator. I felt like I was being played with, and worse, the game didn't even have a purpose (because the secret was meaningless).

Secrets really can't be shoehorned into a story; they must emerge naturally in the flow of it. If you find yourself trying to make a secret 'fit', or dance around it, or putting it there because otherwise there's nothing to look at for thirty pages, your story's already in trouble, and adding secrets won't save it.