Online Textbooks in Financial Economics

An Introduction to Investment Theory is an online textbook "designed for use in a four-week teaching module for master's students studying introductory finance", written by William N. Goetzmann of Yale University, School of Management. Eight chapters covering theories of financial investment decision, risk, portfolio selection, asset pricing, arbitrage, capital market efficiency. Assumes some basic statistical knowledge, but presentation mostly uses diagrams and simple algebra. This link goes to the Internet Archive's copy of the site.

A textbook on trade finance, with eight modules and hundreds of pages of materials, created within Wikiversity by MSU Global for a project funded by the US Department of Education. The first three modules explain different kinds of risk and risk mitigation techniques.