POH library

Finding the right handbook is harder than it should be. A 1978 172N and a G1000 172S share a type certificate and almost nothing else that matters for numbers. This is a directory of where each variant's handbook actually lives, with an honest note on what you are downloading.

Aircraft photography via Wikimedia Commons — full credits on each aircraft's checklist page.

FAA handbooks (free)

U.S. government works, so these are public domain: download them, print them, hand them to a student. No manufacturer copyright to worry about. The FAA moves file paths around, so these point at landing pages where one exists.

Pilot's Handbook of Aeronautical Knowledge FAA-H-8083-25C
The ground-school book. Chapter 10 explains how to read a POH and what each section is for, which is where to start if a handbook feels opaque.
Airplane Flying Handbook FAA-H-8083-3C
Technique: takeoffs, landings, stalls, emergencies. Per-chapter PDFs, because the full book runs well over 200 MB.
Aircraft Weight and Balance Handbook FAA-H-8083-1B
How to run a real loading problem, including what to do when the equipment list in the POH no longer matches the airplane.
Instrument Flying Handbook FAA-H-8083-15B
Instrument technique and systems. Pairs with the Instrument Procedures Handbook for the chart and procedure side.
Instrument Procedures Handbook FAA-H-8083-16B
Departures, arrivals, approaches, and the ATC side of an IFR flight.
Risk Management Handbook FAA-H-8083-2A
Personal minimums and decision-making frameworks. Short, and the most useful thing on this list for a low-time pilot planning a long trip.
Aviation Weather Handbook FAA-H-8083-28B
The consolidated weather reference that replaced AC 00-6 and AC 00-45. Product decoding lives here now.
Aviation Instructor's Handbook AIH
For CFIs and CFI candidates. Also useful if you are building training material or briefing checklists for a club.
PHAK, AFH, and Weight and Balance MOSAIC addenda
The FAA published addenda rather than reissuing the handbooks for the MOSAIC light-sport rule changes. Read the addendum alongside the parent handbook, not instead of it.
All FAA aviation handbooks and manuals Index
The FAA's own index page. Go here first when a direct link below has moved, because they do move.

Building a checklist for one of these? Start from a checklist template and cross-check every line against the handbook you just found.