When it comes to Bond I prefer the films to the books. I'm not sure if the films are still the events they used to be, but I have early memories of being taken to the cinema to see Roger Moore era Bond (he's still my favourite) and how much we loved the special effects.
The only one of the books I managed to get all the way through was Casino Royale, but I do find Fleming's descriptions of food and drinks interesting, and the Vesper (which fortunately for me is in Casino Royale) still gets people to buy Lillet Blanc. Unfortunately it's Kinna Lillet that Bond specifies, which is no longer made, Lillet Blanc isn't as bitter so isn't the ideal substitute.
Hands up, I did not know this before I read 'A Spirited Guide to Vermouth', but at least I do now know that Cocchi Americano (the white one that I need to order online, not the pink one that I can buy in Waitrose*) is considered a reasonably accurate substitute.
There are a few other things that have changed since Fleming wrote Casino Royale - he specifies Gordon's Gin which has dropped its abv. Because the Vesper is shaken, not stirred, you get more dilution, so the change in abv matters a bit. If you can get Gordon's export strength great, if you can't a lot of people recommend Tanqueray, or you could look for a stronger vodka. Bond tells the barman that a grain vodka is better than the potato vodka he's used, so Smirnoff Blue Label would be perfect.
Vodka is another tricky beast, roughly speaking grain vodka will be crisper, potato based vodkas are creamier in the mouth. Fruit based vodkas (Chase do one made from apples, Ciroc is made with grapes as examples) retain a subtle but distinctive hint of their origins. The differences are more noticibly when you taste the products side by side. Personally as long as the abv is at least 40% I'm more or less happy with any mid range brand for cocktail making.
So - a Vesper is a mix of gin, vodka, and Cocchi Americano shaken over ice, and garnished with a strip of lemon. Bond specified 3 parts gin, 1 part vodka, 1/2 a part Kinna Lillet. The Wikipedia entry on the Vesper is worth a read.
*The recipes I've looked up, including Bevan's version in A Spirited Guide assume you know that Cocchi do more than one Americano, and that you will need the one that doesn't have nationwide distribution. I'm not convinced that this is obvious, but I'm pleased to know there is a good substitute for Kinna Lillet out there when I want it.