The UK Toyota Site has put together a nice introduction to the History of Land Cruiser. It covers mostly the early development, but gets into other variations (such as the 70 series, Prado, and FJ Cruiser) that have become the most popular Land Cruiser models.
For a quick read, this is a great start for light & medium duty Land Cruiser history.
The Toyota Global site has done a great job with a fully interactive Land Cruiser History experience. The site includes detailed information on every model including the BJ, 20-30 series, FJ-40, FJ-55, 50 Series, 80 Series, 100 Series, and Prado 120 Series. It's a little dated in that the 200 series & FJ Cruiser models are not included, but overall this is a very thorough site showcasing the ghosts of Land Cruiser past.
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A great graphic showing many of the variations of Land Cruiser over the years. One thing to note is that nearly every Toyota Truck & SUV can trace it's lineage back to the original Land Cruiser.
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Finally, a basic graphic showing the main Land Cruiser models through the 120 Series
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