My collection has crested 1000 titles of bicycle related books.
Not many " Best bike ride in..." either!
There are any number with history and evolution of type set ups.
Most are out of print, while some have been reprinted.
The design books - Sharps Treatise on Bicycles and Tricycles(reprinted at least 3 times)
and The Bicycle Data Book are key.
Books mentioned above are also very desirable.
Beyond that, there are titles on everything related are out there. Finding a hole -
depends on what you like.
As to what I would like to know more about- is the old bicycle industry.
The evolution, and ultimate demise of many of the various companies we all collect.
There were any number of small wholesale vendors around the country.
They had many brands of bicycles they distributed, and many distributed
Schwinn branded stuff as well.
The industry of the 1960's still has a lot of the players hanging in retirement.
Some still involved (Jay Townley for example). A whole bunch of information
in their heads.
I am certain there are more possibilities for bicycle books out there.