Philadelphia’s literary presence peaked in 1940: it’s growing, slowly, shows Google Books

Google Labs did an amazing thing this week.
Mired in copyright battles over its collection of some 5.2 million digitized books, the company released the 500 billions words that lie within, without context other than its published year. In one of its forms, the use of any word can be graphed from 1500 to 2008 or at any limit in between. This has, of course, brought on a wave of discussion and research.
For us, well, it’s all about Philadelphia. And, in graphing Philadelphia, it becomes clear that during World War II, Philadelphia was a hot topic. In a story we all know too well, the next half century wasn’t particularly kind to Philly, including de-industrialization, white flight and, yes, a crash and subsequent flat-lining of cultural references to our metropolis.
Who has an explanation for that explosion in the 1940s? Could it be misleading data, considering Pittsburgh goes through such a similar blast in the late 1950s and early 1960s?

Without any research backing the meaning, such graphs can be misleading, but, it is interesting to see Chicago’s rise of late and the spikes of San Francisco. [Note: Boston and New York are not included because, despite their varied population sizes, the number of their references in publishing is quite a bit larger than these other large cities, even when accounting for the state name.]
See the same cities from above stretched in a graph from 1740 to 2008, to see the impact the Revolution and other spikes.
Other Philly-related graphs we thought were cool:

• Philly versus Philadelphia
• Philadelphia versus Houston since 1900 — Note the surge after the oil boom there.
• Philadelphia versus Pennsylvania versus Pittsburgh since 1900 — Again, why do Philly and Pittsburgh have such similar bursts?

What cool Philly graphs are we missing? Any explanations for that Philly spike?