Bogleheads Wiki
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Community-maintained wiki from the Bogleheads investing forum explaining how to build and maintain a low-cost, diversified portfolio of index funds. Covers asset allocation, lazy portfolios, rebalancing, tax-efficient fund placement, and risk tolerance, so readers can design and run a passive portfolio themselves.
More resources on Portfolio Management
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.
Portfolio Visualizer
Web-based toolkit for backtesting asset allocations, running Monte Carlo simulations, factor regressions, and efficient-frontier optimisation against historical fund and index data. Lets investors test how a proposed portfolio would have behaved through past drawdowns before committing money; some features require a paid plan.
Investment and Portfolio Management
University of Geneva course in the Investment Management specialization covering modern portfolio theory, asset allocation in practice, and risk measurement. Learners finish able to construct a diversified portfolio, compare allocation approaches, and measure and manage portfolio risk.
The Intelligent Asset Allocator
William Bernstein's short, quantitative case for building a diversified portfolio: how correlations between asset classes reduce risk, how to estimate expected returns, and how to choose, weight and rebalance index funds into a long-term allocation.
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 Four Pillars of Investing
Bernstein's guide organized around four areas every investor needs: the theory of risk and return, the history of market manias and crashes, the behavioral traps investors fall into, and the business practices of the fund industry, ending with how to assemble a portfolio.