It's possible to write a book with just a working title, but I think the sooner you have the title, the better -- and the easier. I waited too long, I think, to settle on the title for The Grand Mirage and it was hard at that point to actually figure it out (and then retrofit the title into the book).
Working titles are easy. They are one word and it only has to speak to me. Mirage, for instance, was called Orient, which told me what I needed to know but couldn't be used in the title because most people now associate Orient with the Far East. I thought of Orientalist as the title, but that didn't really solve the problem and the biography of Kurban Said had just come out with that title.
So now I'm working on the sequel to Mirage -- or really the next in the series. My first idea was one set in Cairo (working title: Cairo) and involving oil, tentatively titled Black Sands. I liked that because, as with The Grand Mirage, it directly evoked the desert. I may get back to that but in the meantime I feel more inspired to have the next one set in the Levant (working title: Levant, you see how it goes). Unfortunately, as I've noted in a related blog, this concept doesn't seem to be commonly known in the U.S. So any title like Shadows on the Levant would not resonate the way I would like.
I'm beginning to understand why many of Alan Furst's titles are so vague (Night Soldiers, Kingdom of Shadows, Red Gold, The World at Night, Dark Voyage). They are evocative but evanescent and I'm sure I'm not the only one who can't remember which title goes with which plot. I can't even remember which ones I read. Some of the others (Spies of the Balkans, The Spies of Warsaw) are literally easier to place, but betray a certain lack of imagination. His forthcoming book apparently is titled Mission to Paris.
In fact, one of the titles I considered for Mirage along these lines was Caravan to Baghdad. So in the sequel I should forget Levant and think about the names of some cities involved, such as Damascus and Beirut (Smyrna and Alexandretta probably don't have sufficient name recognition). Combining this with some evocative concept like "shadows" or "winds" may do the trick, but something more concrete may be called for. We'll see.