The Department of Education's (DOE) website is a good place to start for information about the process: http://schools.nyc.gov/ChoicesEnrollment/High/default.htm
The DOE's High School Directory is the source of school facts (keep in mind, there have been known to be some errors in the book): http://schools.nyc.gov/ChoicesEnrollment/High/Resources/default.htm
The DOE now has a high school event calendar: http://schools.nyc.gov/ChoicesEnrollment/High/AdmissionsCalendar.htm
NYC Schoolfinder is another resource many parents take advantage of: http://schoolfinder.nyc.gov/#/filters/order:alpha
(keep in mind this is a beta version and is a work in progress).
There's also the NYC School Search at: http://schools.nyc.gov/SchoolSearch/?advanced&hs which helps you locate schools by zone, borough, admissions method, and more.
The DOE put together a Schools Quality Report site, that makes it easy to up up stats for each school: http://schoolqualityreports.nyc/reports/dashboard.html
You can check out individual school's Fair Student Funding percentages at: http://schools.nyc.gov/AboutUs/funding/FSFDetail/default.htm
If you're looking for SHSAT handbooks, you can check some out at: https://www.amazon.com/s/ref=nb_sb_noss?url=search-alias%3Daps&field-keywords=shsat+guides