Sheldon Brown is a name every cyclist should know. His website, at www.sheldonbrown.com/ is an encyclopedia of anything useful you might wish to know about bicycle mechanics and history.
Sadly he passed away this week, to much sadness among cyclists who have relied on his practical wisdom to resolve and explain technical issues which the manuals rarely manage to do so clearly and with so much enthusiasm for their topic.
I’ve relied on his online gear calculator on numerous occasions when trying to figure out what casette to buy and what the advantages are of a compact as opposed to a triple and so on. His site is full of those sort of things that others fail to break down to the simple level of the end user. Hopefully it will remain long after his passing.