The ERISA Outline Book by ASPPA provides the most comprehensive details. The hard copy series is generally on sale at the end of the year, and it's still a good reference tool after a number of years, even though a new version is published annually. I recommend also staying current with IRS releases such as Revenue Rulings and the Federal Register on the IRS website. The Pension Answer Books are good too, but it's more college 101 for what we do compared to advanced with EOB.