Some writers use secrets in an attempt to keep the reader hooked. The main character has this big secret, and you need to stick around to find out what that secret is. Some readers like that.
I'm not one of them.
I actually hate it when the main character has a secret. Hate it.
This is the way I see it. When the main character has a secret, it feels like I'm playing a game with someone who is playing by a different set of rules than I am. The deck is stacked in the writer and the main character's favor. They're hiding their cards. They both know what cards they have, and they're showing them to each other and laughing at me because they know something I don't know. And I don't know it, because they're the ones making the rules, and their rules say I can't know this "big secret," which is apparently a secret only to me.
I find this particularly annoying when the book seems to be about nothing other than the secret. It's the sort of thing that can get me to close a book within the first few pages. The writer has to show me all of his or her cards. If not, I'm going to find another writer who will.
I'm okay if the main character's secret is one the main character doesn't know, for example, if the main character is in denial. That's okay. I get to see all the cards the main character can see, and that means the rules of the game are fair.
The best thing to keep me hooked, though, isn't a secret at all: it's the need to find out how the main character will resolve his or her conflict.
Make me invested in the main character. Make me want what the main character wants as much as the main character wants it, and make me need to find out how the main character does or doesn't get it. That's all a writer needs to keep me hooked.