The Oxford Wine Companion and The World Atlas of Wine are both encyclopaedic and contain good info, but are not well set for use as a single reference imo. I've seen groups of Wine Guild Education students comparing strategies on how to reorganize the content of these two books into usable formats. They use these books as their standard reference material but have difficulty using it as presented. I have them but rarely consult them.
For easy reference and one book, I prefer Sotheby's Wine Encyclopedia. Not everyone agrees with Tom Stevenson's reviews of individual wineries, but it's well organized and easy to use as a reference. You can also read through sections on various regions and get an indication of the region.
Each book has its appeal and drawbacks. A good strategy may be to go to a book store and check out the short list from the recommendations here and see which you prefer. They all have good information in them.