A few points to keep in mind.
1. Oil is finite in human scale. (i.e. taking hundreds of milliions of years to create)
2. Even the optimistic forecasts predict a peak in oil production at 2030-50. It is a given, peak oil is a when not if argument. NOTE: There will always be billions of barrels of oil around.
3. US oil production has peaked, back in 1970-71
4. There are other notable peak oil mentions. Mexico and it's Cantrell field peaked, UK/Norway north sea peaked, Kuwait's Burgan field peaked, Australia peaked, Indonesia peaked, Russia peaked, Saudi Arabia's Gawhar (appears to be plateaued or peaked)
I would suggest reading some of matthew simmons (google the name he's the investment banker) and his presentations.
The only person I would really avoid is Kunstler, he is more rant oriented and is likely to put a newcomer off, however for old timers it can be humorous to read.
I also like Richard Heinberg and his take on energy consumption.
Another great website for introduction is called The Wolf at the Door. If you are a math head The Oil Drum is an excellent source of highly detailed technical analysis on oil depletion.
I would not spend excessive money on books, most of the information is available free of charge.
And as to why the second half of a fields oil is more difficult to extract. First it has to be known that most fields will typically contain greater than 40% of their original reserves, even after it stops producing. As there is simply not enough energy to push the oil (various grades sweet, heavy, sour) through the various rock formations.
The reason the second half of production is harder is very basically something this.
If you have water in a sponge getting the first half of the water out is much easier than the second half. And there will always be water left in the sponge.
Thats why we won't ever run out of oil 