Is your plug wet? Also loading up a motor with starting fluid especially either is not a good idea. Use it to diagnose your symptom. You don't need but a tiny snort of it to make the motor fire up.
You can also put a teaspoon of gas into the cylinder when replacing the plug and if it was missing fuel from the equation it will fire up once.
Your book is totally right about the spark needing to be hot to get into a compressed cylinder. In some cases like with a cracked plug you will see a nice spark outside the cylinder but not know whats going on when it is installed. Also if the spark circuit is breaking down bad cdi or wire any connections this thing can happen.
These are neat checkers as they force the spark to jump across a predetermined measured gap
Thexton 404 Ignition Spark Tester with Adjustable Gap - Toolfetch.com
For now lets just keep it simple. Is your plug getting wet? How long has this motor been in storage? Yes gas goes bad with age sitting in storage. A carb can get all gummed up to where the venturi jets inside etc will plug up.