Although it has become something of a stereotype of the modern fantasy novel to include a map at the front of the book, the idea is far older, a classic example being the Wessex map featured in the Macmillan editions of Thomas Hardy novels. This semi-fictionalised southern England - the landscapes stay the same, but Bournemouth becomes Sandbourne, Dorchester Casterbridge, and so on - has become iconic of Hardy's vision, and there are lots of very nice books exploring the relationship between the fictional and real locations. Just skimming the shelves here, I find Desmond Hawkins' Hardy's Wessex; Kenneth Phelps' The Wormwood Cup: Thomas Hardy In Cornwall; and the Penguin reprint of Hermann Lea's two-volume The Hardy Guides, a 1913 travel guide prepared in collaboration with Hardy.
It's tempting to think of Wessex as a fully-fledged mental landscape within which Hardy wrote. Yet this is far from the truth : Birgit Plietzsch's research project Thomas Hardy's Wessex traces through successive drafts and correspondence how the unifying 'Wessex' and its frontispiece maps grew as a marketing concept that Hardy himself initially resisted. More maps, and on Hardy in general, at the Thomas Hardy Association website.
Topsham in Devon, incidentally, has a minor Hardy connection: Hardy's cousin Tryphena Sparks (with whom he may have had a romance in his younger days) married a publican here, and is buried in Topsham cemetery.
Addendum: I just ran into another related book, Hardy's Wessex Today (Mellstock Press, 1990). This is a nice little paperback, the first volume - I don't think there have been any more - in Vera Jesty's project to collate the 240 Hermann Lea photos of Hardy locations with her own 1985-1990 ones.