What's the single best piece of financial advice you've ever received?

The best financial advice I’ve ever gotten is:
'‘It doesn’t matter if you now buy at the top, bottom, or somewhere in between. Just getting in the market is what counts.’'
My dad told me this when I started investing, and it’s been solid advice. So many (including me back then) people stress over timing although they plan to stay in the market for a long time.
What’s the best financial tip you’ve ever gotten, and who gave it to you?
Also, any book that’s really helped you? For me, Bogle’s Common Sense on Mutual Funds and Poor Charlie's Almanack has been great ones in their own different ways.