Prewar:
Book Publication: Greenberg's Guide to Lionel Prewar Parts & Instruction Sheets
Digital Publication: Olsen's Toy Train Parts has many digital CDs
Postwar:
Online: The LCCA/HSL Digital Archive is available to LCCA Members for free and covers all Lionel postwar items.
Book Publication: Greenberg's Repair and Operating Manual for Lionel Trains, 1945–1969
Digital Publication: Olsen's Toy Train Parts has many digital CDs
MPC/Modern Era:
Online: Lionel - https://www.lionelsupport.com/service-documents/ Product Suppliments 1-47
Book Publication: Greenberg's Lionel Modern Era Diagrams and Parts Lists (Covers through ~1991)