The True Meaning of Smekday by Adam Rex, 423 pages, fantasy.

My 2011 review (we listen to this every year or so):
I read this a couple of years ago and loved it. It has taken me a long time to get my kids to agree to listen to it. I'm glad that we listened to the audio, because Bahni Turpin does a fabulous job and has become one of my favorite readers. She does a great J-Lo (the invading Boov have unpronounceable names and may take common Earth names to put us at our ease). Twelve-year-old Gratuity "Tip" Tucci writes a school essay about what Smekday means to her in the post-invasion days of 2013-2014. She recounts her adventures with J-Lo and their Booved-auto, "Slushious," as they travel across the land, avoiding Boov and Gorg, and trying to find Tip's mom.
Layered, weird, and funny, Smekday is great. Since we first listened to it, it has become a family favorite. It has also been made into a less-than-stellar movie.