Bogleheads Forum
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The main discussion board of the Bogleheads community, where amateur and professional investors debate portfolio construction, tax placement and fund selection. Long-running threads and a strong norm of low-cost passive investing make it a place to sanity-check a plan.
More resources on Index Fund Investing
Bogleheads Risk Wiki
The Bogleheads wiki page cataloguing the risks a long-term investor faces: market, inflation, interest-rate, currency, and behavioral. After reading it you can distinguish risks diversification removes from those it cannot, and size your allocation accordingly.
Bogleheads Wiki
A community-maintained reference covering three-fund portfolios, tax-efficient fund placement, retirement account types and withdrawal strategies. Reading the core pages gives you a complete, vendor-neutral plan for building and maintaining a low-cost index portfolio.
Investing Essentials
Learn index fund investing with this course from the Khan Academy Team! Master the essentials and build a solid financial future.
Passive Investing with Index Funds
An Indian School of Business course for learners with no finance background, moving from reading corporate financial statements to risk, diversification and portfolio construction. Four modules also cover time value of money, compounding and how securities trading works.
vanguard.com
Vanguard.com is the official site of Vanguard, a leading provider of low-cost index funds and ETFs. It offers fund listings and performance data, plus educational content, planning tools, and resources for learning about passive/index investing.
The Bogleheads' Guide to Investing
Written by three long-time members of the Bogleheads forum, this covers building a low-cost index portfolio, asset allocation, tax placement, and the behavioural traps that wreck returns. Finishing it leaves you with a workable lifetime investing plan.