A book for when you're lonely
Every reader knows that a book becomes a friend, and every traveller knows that the book you take with you becomes your travelling companion. For good or bad, the voice you hear in your head is that of the prose you've just been reading. You decide - as you sit squashed on the bus is it Paul Theroux or Dervla Murphy you want to hear narrating the experience? Best of all is to have a travelling companion who's visited the places you're seeing. My travels with Edith Durham's High Albania and Through The Land of the Serb formed the basis of my book, Edith and I; on the trail of an Edwardian traveller in Kosovo.
A book for when you're homesick
However exciting the journey, sometimes you want to escape to somewhere where everything is familiar. It's the same urge that found me boiling up porridge in a Jerusalem youth hostel kitchen, but Agatha Christie can be comfort food in just the same way. Bringing something from home - whether it's Christie or JK Rowling or Monica Ali - can also work like having a photo of your family with you to share your world with those you meet.
An audio book
There's a power cut. Or you're on an overnight bus and everyone else is sleeping. Or you just need to save space in your luggage. An audio book has the comfortable intimacy of your mother reading you a bedtime story, and weighs nothing on your iPod. Just don't be tempted to read Archangel on the overnight train from St Petersburg to Moscow - when I was so rigid with fear as we raced through the ghostly birch forest inhabited by the book's most terrifying monster that I couldn't even bring myself to move the hand necessary to press the pause button.
A page-turner with short chapters
You need this when you're on the move - particularly through the stop-start of airport procedures - or facing long but interrupted waits at banks or embassies; a book that can be easily picked up and put down.
A book with some phrases of the local language
This doesn't necessarily mean a phrase book, though these can be useful too (or sometimes wonderfully useless. My favourite Kosovan phrasebook gives translations in Albanian, Serbian, Turkish among other languages for the essentials like 'I'm sorry but we have no bread/ money/ guitars' and 'tomorrow is the circumcision of my brother's son'). One of the few words of Romanian I know is the word for rose, mentioned in The Fortunes of War as the name of a popular Bucharest restaurant. I was able to deploy it to acclaim on my trip to Transylvania.
The Economist World Book of Figures
This slim volume is my constant travelling companion and all-time winner for minutes of entertainment (not to say education) per gram of weight. Find out the intimate details of the country you're travelling, and beyond. How much time does the population spend eating per day? What percentage of women here use modern contraception? What are the rates of TV ownership? Death by traffic accident? How have you never thought to ask these questions before?
A book you can leave behind
Whether through the organized community of www.bookcrossing.com or the more spontaneous co-operatives of hostels and hotels, having a book you're happy to leave behind is a connection between you and the others passing through - and is your library ticket for borrowing from an endless chain across continents. Just like the guy who traded in a paperclip for a house on eBay, you can swap your light novella for a modern bestseller in one town, the bestseller for a Booker prize winner in the next, and so on, and find yourself with Ulysses in your rucksack by the end of your trip.
A book you can write in
This doesn't have to be a notebook; as long as you're happy with writing notes in the margins, any book can become a notebook. In fact, with ticket stubs used as bookmarks, wine stains on the page you were reading in the bar, and the jottings of how much you've spent on the inside back cover, every book becomes a scrapbook of your travels. An evocative whiff of cheap cigarettes or wood smoke comes off their pages when they're opened up back home.
A book of poetry
Good poetry is reread and savoured, so you get double the value of prose out of the pages of a poetry book. If you can find something written in or about the country you're visiting, you'll appreciate it even more.
A book with lots of pages
Not everyone is as bold as my companion on our round-the-world trip who set off with War and Peace and ripped out the pages as he went so that by the time we were back in England he was carrying the light pamphlet-length finale to the story. Sorry to say, however good the text is, sometimes you value a book for its paper. Perhaps everyone has had their own version of my Varanasi bout of diarrhoea when in the fetid dark of a squat toilet, my Rough Guide had to yield up its contents pages in a more urgent cause.
Elizabeth Gowing's latest book, The Rubbish-Picker's Wife; an unlikely friendship in Kosovo was published this summer. More at www.rubbishpickerswife.com