The Bhagavad Gita of Multidimensional Access Methods...
The Koran of Quadtrees...
One of the people I met at ACM GIS 2007 was the conference chair, Hanan Samet. He's published a series of highly useful books on spatial indexing methods, which are certainly the most comprehensive and detailed in this field. His latest work is Foundations of Multidimensional and Metric Data Structures and is surely his magnum opus. It could also be called Everything you wanted to know about Spatial Indexes (but were afraid to code). Recommended reading for spatial geeks, especially if you're looking for some reading to tide you over during your next year off.
His first book on the subject, The Design and Analysis of Spatial Data Structures, is also worth looking at, especially since it's a bit more bite-sized. However, it's out-of-print, and expensive when you do find a copy (local plug: I found mine through ABE) If you really can't find a copy you might try contacting Hanan directly.
Hanan has a clear preference for quadtree structures over R-trees, but both approaches are covered well (including pseudo-code - yay!)
Keynote Talks from OOP
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