Send It Snail Mail
As novelist Maureen Johnson explains, emails "can come in with such ferocity that several authors I know have removed their email addresses from public view ... because there is simply too much (to deal with)." Sending a letter through the post office makes your letter stand out, shows you care enough to take a little time sending it, and may increase the chances of hearing back from the author. Occasionally, authors include a mailing address on their websites. If not, write to the author in care of the publisher, who should forward your letter to the author.
Use a Personal Format
When writing to authors, use a personal letter format. Include an opening salutation and a closing. The first paragraph should briefly explain who you are and why you are writing. The paragraphs that follow should go into further detail. It's fine to share personal details that apply to the book; for example, if you just had a bad breakup and the book helped you through it, mention this. But don't ask for favors, and keep the letter short and positive. If you hope for a reply from the author, include a stamped envelope addressed to yourself.