I have never read any of his books.
Will be on the road in a couple of weeks, may have to check one out.
If you want to get the flavor before you read the two best, you could start with the book just before those, "The Spy Who Loved Me." The last book written by Fleming, but completed after his death, was "The Man With the Golden Gun," which followed "You Only Live Twice." It's great, too.
So the order is...
"The Spy Who Loved Me"
"On Her Majesty's Secret Service"
"You Only Live Twice".
"The Man With The Golden Gun"
Here's Bond's fictitious obituary written for "You Only Live Twice"
And here's a great website that gives a lot of information such as a quick synopsis of each book along with the great cover art of the early books.
From that same site you can get a lot of good information on Fleming himself and his books.