I can also recommend the book
'The World without Us', which was written by the author of the original thought experiment article on which the 'Life after People' series was based.
In it, he elaborates on the subject, basing his findings on thorough research. It even has a pretty comprehensive source list in the back.
Basically, he describes the physical processes involved in the gradual decay of all human products, be they skyscrapers, roads, ships, crop fields or thoroughbred horses, should they all be abandoned at once.
I found it a really entertaining read and it should spawn quite some ideas.
Might I also recommend the S.T.A.L.K.E.R. games? The urban levels should provide some ideas (Call of Prypyat especially). Also, the 'now' above ground scenes in 12 Monkeys, and maybe also The Road (can't judge; I have it on DVD but haven't seen it yet), maybe the original Robocop movie too; after all it's set in a near future, decayed Detroit...