Haven't seen this mentioned yet, so:
You can write care of the publishing company.

As others mentioned, nothing beats a handwritten letter. But you don't want to go into stalker mode to find the writer's address. Publishing companies will absolutely forward the authors' mails. They might open it first, to screen the weirdos. So, don't write anything horrible, avoid nudie pics (how is that even worth mentioning? anyways), be positive and your letter will reach the writer. Don't send anything else though, like an object, even if it means a lot to you that this writer should receive that object. Red flags.

And yes, speaking for me personally, it's always great to receive a nice message from a reader. Unless you're a superstar, writing is a lonely business.