Odd couple coauthors Michael Ian Black and Meghan McCain drew a decent crowd to the Brattle Theatre Thursday for a signing of their new book, “America, You Sexy Bitch: A Love Letter to Freedom.” (The signing was arranged by the Harvard Book Store.) Black, an actor and comedian best known for his commentaries on VH1’s “I Love the 70s/80s/90s/New Millennium” series, took a cross country road trip with Senator John McCain’s daughter and then wrote a mildly humorous book about it.