Permission

I don't know if anyone's had any experience with this but I'm hoping someone will be able to help me out. In my book I want one of my characters to be reading "To Kill a Mockingbird" in school, so my question is do I need permission for this?

Obviously if it's ever published I will but do I need permission before I start sending it out to agents?

A friend of mine said I should avoid it altogether but I really like the part that I've written and it bums me out that I could have to cut it.

Wise to avoid mentioning real world things for a first book, but you can get away with it, heck if you name drop the right way they might even pay you extra for the advertisement.

Still I say avoid it unless its something so old nobody is alive to claim ownership of it. That book is still being published under the original publishing company only, so I think its still claimed. So might be best to make a light reference or create a fake book that is similar.

I did some research online and it said unless I directly quote from the book, which I didn't, you don't permission. I plan on sending my book to agents so I feel like if I need permission they'll help me out. Thanks for the advice!