On the subject of Midi-chlorians; Having read lots of SW related material over the years, it seems that Lucas had a plethora of ideas that weren't used for financial reasons or were dismissed or discouraged by those around him. In creating a universe and characters it's clearly important to have some background as a reference. Remember Oz writing his own Yoda background to help him act the part? The point is that the background doesn't have to be relayed to the audience.
I just think it's a shame that many of Lucas's rejected ideas were recycled for the PT. Many of them were previously dismissed for good reason.
Just in the same way that explaining how a magic trick works, giving us a 'canon' description, it takes away the impact and mystique. For me, Lucas managed to strip away 23 years of mystique in about 2 hours by mapping out the science of The Force and making all those weak connections between the characters of the PT and OT.
One thing that this book does well is avoid making these connections. It purely looks at the conception and creation of the first Star Wars film. I'll shut up now.