Even though this is widely-known information, for reliable source you can refer to Making of ESB by Rinzle
Those books aren’t as “reliable” as you may think. People who worked on those movies have stated that they are basically the “Revised history according to George Lucas”.
Who are those people and what are the prime examples of revision? The book pretty much focus only on reliable sources, such as: actual material (scripts, concept art, footage, etc.), interviews with involved (lots of those are from the time films were made/released) and transcripts of various meetings.