Ephemeral New York chronicles the city's history through old photos, artifacts, and local stories, written in a short post format.
HN commenters call it terrific for New York history and link its posts on the Chrysler Building's colonial road and noisy 19th-century city life.
It's remarkably hard to find images of what it used to look like. I've seen some before. Here's what one section looked like in the 1870s when the railroad run underneath it:
http://www.iridetheharlemline.com/wp-content/uploads/2010/05/f1.jpg
Here's park avenue in the 1920s:
http://screencast.com/t/7GRMhOzq
Park Avenue (used to be 4th Avenue) has probably changed more than most Avenues in NYC. Here are some images of when railroads ran down it.
http://ephemeralnewyork.wordpress.com/2013/01/28/three-views-of-park-avenue-in-the-east-50s/
You may also enjoy seeing these photos of the NYC mansions from back in the day:
http://ny.curbed.com/archives/2012/02/09/looking_back_at_manhattans_lost_gilded_age_mansions.php
Arguably, that process has already started. To anyone who was born after 9/11 (and that was almost 18 years ago, so there are people born after 9/11 who are almost old enough to vote!), that event is already kind of an abstraction, in much the same way that the attack on Pearl Harbor is to almost everyone reading this. The passing of an event out of living memory is the first stage of that kind of transition from experience into legend.
And also, if you're interested in this sort of thing, I recommend subscribing to the blog Ephemeral New York, which highlights all sorts of still-existing traces of New York Cities long gone: https://ephemeralnewyork.wordpress.com/
For a little more context, about 40% of the buildings in Manhattan couldn't be built under today's zoning laws: https://www.nytimes.com/interactive/2016/05/19/upshot/forty-percent-of-manhattans-buildings-could-not-be-built-today.html
In other words, the very thing that people move to NYC for can no longer be built in much of NYC. Pretty perverse.
In the 1950s, nice apartments in Greenwich Village or the UES might be $800 – $1,500, adjusted for inflation: https://ephemeralnewyork.wordpress.com/2013/09/02/what-a-west-village-apartment-cost-in-1955 .
What changed between then and now? Zoning: https://jakeseliger.com/2015/12/27/why-did-cities-freeze-in-the-1970s
The website Ephemeral New York is terrific for all sorts of interesting New York history: https://ephemeralnewyork.wordpress.com (the New Year's day post is where I found the reference to her diary)
And the Historical Atlas of New York City is both visually and textually superb for all sorts of fascinating details of New York's development: http://www.amazon.com/Historical-Atlas-New-York-City/dp/0805078428
Finally, "Letters of Note" has great finds from historical letters throughout history: http://www.lettersofnote.com/
That reminds me of the trapezoidal Chrysler Building[1] in NYC, shaped by a plot boundary reflecting the pre-grid Boston Post Road[2] (only 2 weeks NYC-Boston!). The Road elsewheres reflects 1600's colonial improvements to the earlier Pequot Path, and influenced US1 et al.
[1] https://ephemeralnewyork.wordpress.com/tag/chrysler-building-boston-post-road/ [2] https://en.wikipedia.org/wiki/Boston_Post_Road
Yes, I remember hearing that they had to convince the public it was safe by having elephants cross it first.
I looked it up, and it turns out that there was an elephant march, but it was delayed until long after the (for-humans) opening, but still was a big spectacle:
https://ephemeralnewyork.wordpress.com/2011/12/22/the-elephants-that-tested-the-new-brooklyn-bridge/