
If you've got 10+ years, keep it mostly in stocks. If you need it sooner, keep it mostly in bonds. Trust in the power of compound interest.
Prefer assets that have a logical reason to go up based on history, not fancy. Businesses and governments pay back bonds from their revenue. Businesses are worth more over time from improvements in efficiency and scale which raises the stock market. Stay away from assets that are basically speculative (gold, crypto, oil, metals, real estate). History shows that speculative investments tend to just track inflation over time, but since they're noisier people can find all kinds of short term patterns to justify investing. Don't fall for it.
If I had to recommend _one_, it would be Four Pillars. The first part is a bit more intellectually "challenging" than Random Walk, but ultimately does a better job (IMO) at backing up the author's ideas on personal investing with data.
Random Walk covers all the major bubbles in history, and does an excellent job at breaking down asset allocation per age range in the later sections of the book.
All About Asset Allocation was very useful to me in developing a long term investment plan, and deciding what asset classes to include in my portfolio.
I refer to each book at least once a year.
General Investing
# The Four Pillars of Investing - by William ("Bill") Bernstein.
# Wise Investing Made Simple or The Only Guide to a Winning Investment Strategy You'll Ever Need - by Larry Swedroe
# The Bogleheads' Guide to Investing - by Taylor Larimore, Mel Lindauer, and Michael LeBoeuf
# All About Asset Allocation - by Rick Ferri
# The Informed Investor - by Frank Armstrong
# The Little Book of Common Sense Investing - by John "Jack" Bogle
# The Coffeehouse Investor - by Bill Schultheis.
Investor Behavior
# Why Smart People Make Big Money Mistakes And How To Correct Them - by Gary Belsky and Thomas Gilovich
# Your Money and Your Brain: How the New Science of Neuroeconomics Can Help Make You Rich - by Jason Zweig
# Rational Investing in Irrational Times: How to Avoid the Costly Mistakes Even Smart People Make Today - by Larry Swedroe
Financial History
# Against the Gods and Capital Ideas - by Peter L. Bernstein
# Devil Take the Hindmost: A History of Financial Speculation - by Edward Chancellor
# A Random Walk Down Wall Street - by Burton Malkiel
Like others here have mentioned, http://www.bogleheads.org/ is a great forum for Vanguard investors.