I can't say that I've seen any literature on the art of floorplanning. Unfortunately really good floorplanning is more of an art than a science. That should not discourage the neophyte however, as even basic floorplanning can have dramatic results. The goal is of course to place the logic in ways that make the routing easier, less congested and shorter. As a starting point, you might let the tool do the place and route. After it is finished, read the placed design into the floorplan tool and start looking for ways you can improve the layout. The first thing you will probably notice is how awful the automatic placers really are.
For Xilinx designs you will have to get a hold of XACT6. There is a beta floorplanner for M1.4, but it is really not ready for prime time yet. In my opinion, there is not enough info supplied with the beta floorplanner for someone who is not already familiar with the xact6 floorplanner to use it. Anyway, get a copy of xact 6 and look at the on-line documentation for the floorplanner. Play with the tool using a simple design (a design with some Relatively Placed Macros and loose logic is probably the best) with an eye toward minimimizing the complexity of the interconnect. The Altera MAX PLUS tools also have a floorplanner, which I think is a little harder to use than the xilinx one. Fortunately, the routing structure on the Altera device makes it less sensitve to having a good floorplan (and less capable for really high performance stuff).
Beyond playing with a few designs, I can't really offer any quick advice. Floorplanning is rather like putting together a jigsaw puzzle, except that there are a large number of solutions. Because of that, it takes a little bit of an artist's eye to do it well. When you get down to it, I find that some people have a knack for it, while others just don't. To be honest, most of us who advocate floorplanning have been doing it long before the floorplan tools were available (we used up a lot of graph paper and pencils). Play with the tools on as many designs as you can. As you gain familiarity with the architecture and the tool, you will start to recognize what works and what doesn't. As with the arts, there is no substitute for natural talent. Fortunately, engineers tend to be good puzzle solvers, so there is hope. I'm sorry I couldn't offer more help than this.
Fliptronics, another FPGA consulting firm, has a decent introduction to floorplanning on thier web-site.