I hope you'll forgive me if this post is a little self-indulgent. I'm jumping on a plane tomorrow to fly to New York, and so I thought I'd talk about the books I've chosen to take with me. It's something I think very carefully about. There are guidelines (often ignored) and goals (frequently missed).
The ideal travel book, in my opinion, is a mass-market paperback of middling length. Too thick, and it'll be a nuisance to carry. Too thin, and it'll be over before the plane has boarded. Thin trade paperbacks are okay, but hardcovers are right out. It should be engaging, but not all-consuming, so that it doesn't drag too much of my attention away from my surroundings. I want to enjoy it, but I also want to feel like it's okay to leave it behind when I'm done, in a youth hostel or train station, for someone else to enjoy.
I think the only time I've ever hit all these things perfectly was with The Lies of Locke Lamora  by Scott Lynch.
I tend to prefer authors I'm familiar with, so that I know what to expect. There's not much worse than realising an hour into your flight that you hate your book. Having said that, I always include a complete newie in my luggage, in the hope of a pleasant holiday surprise. I also like to bring a mix of genres, although I generally stick with (old reliable) SF for my actual airplane books.
How many books to pack depends entirely on where I'm going. If I'm travelling to non-English speaking countries, I pack more. If the explicit purpose of the travel is relaxation (on a beach, say), I pack more. If I'm going somewhere like New York, where it'll be easy to find things to read, I feel like I can get away with fewer. Bare minimum is four books, and I've carried as many as seven.
So what have I packed this time? In my hand luggage, I've got two books: In the Mouth of the Whale  by Paul McAuley, and Undertow  by Elizabeth Bear. I'm actually faintly surprised that I chose the McAuley. Though they should be right up my alley, I have a somewhat troubled relationship with his novels. I think that's why I'm carrying the Elizabeth Bear as a backup.
In my checked luggage is The Kingdom of Gods  by N. K. Jemisin, and The Intuitionist  by Colson Whitehead. I'm reading the former because it was nominated for this year's Nebula Award. Actually, I'm reading it because I enjoyed the previous books in the series -- The Hundred Thousand Kingdoms  and The Broken Kingdoms . I'm reading it now because of the Nebula Award. And The Intuitionist is my wild-card. I knew as soon as I heard the premise -- the Intuitionists and the Empiricists, competing schools of elevator inspectors! -- that I had to read it.
I came very, very close to packing Last Call  by Tim Powers, but I ran out of space. I still wish I could find a way to fit it in.
So what's your system for choosing travel books?