The series is the Chronicles of the Imaginarium Geographica by James A. Owen (coppervale) (and I keep misspelling the series name as Geographia :/). The original premise is that three men (Jack, Charles and John) are tapped to be the new caretakers of the Geographica, end up going to a fantasy world to save it from the Winter King. If you are a map geek, or perhaps recognise that combination of names, you might like it. Simon and Schuster have created a lovely site, with wallpapers, audio excerpts and (until Monday), you can read the first book, Here There Be Dragons for free online.
I'm going to cut here,because I just read the fourth book, The Shadow Dragons and the following contains spoilers for the mystery of the first book.
I was a little disappointed with the previous volume, where Charles was sidelined for an alternate version, but this more than made up for it. We get to meet more of the previous caretakers (Kipling!, Shakespeare!, Twain!), and the author follows his pattern of throwaway references to both things that show up in the various authors' work and elements that hint at future caretakers. I like that not everyone who knows the secret of the Geographica agrees about the best course of action. And I love the illustrations (done by the author himself).
The only problems I have thusfar is that there are very few female caretakers (Mary Shelley is one) and that they're heavily skewed towards Western culture - though both are explained by there only being three caretakers at a time and previous caretakers choosing their own successors - it makes sense that they'd choose people within their own social circles and it's clear by the end of this book that the circle will be widening soon.
Still, this book does leave me wondering about when we'll see references to Manly Wade Wellman, Mrs. Molesworth, E. Nesbit (though there might have been a Psammead in one of the earlier books) Lord Dunsany, James Branch Cabell and certain other early British and American fantastic fiction writers. Also I keep expecting to see a time traveler pop out of a blue box, but that may just be me. ;) I did love the fact that they travel by trumps this time around.