Paul uses "in Christ" and its variations many times in his epistles. What main idea(s) is he trying to convey with this phrase? Here is one example to consider:
2 Co 5:17 (KJV)
Therefore if any man be in Christ, he is a new creature: old things are passed away; behold, all things are become new.
Here are several other uses of the phrase in Romans (KJV), but you may focus your answer on the above to simplify:
There is therefore now no condemnation to them which are in Christ Jesus, who walk not after the flesh, but after the Spirit.
For the law of the Spirit of life in Christ Jesus hath made me free from the law of sin and death.
Nor height, nor depth, nor any other creature, shall be able to separate us from the love of God, which is in Christ Jesus our Lord.
My gut impression from reading these (and many other) "in Him" passages is that we in Christ Jesus are like being in a room in which God has stocked everything we'll need for life and godliness, everything we'll need to make it to the end in glory and honor. If we abide in Christ, we'll get the utmost in fruitfulness; our fruit will be tended to the fullest it can be by God. We can access any of them by faith. I'd love to know of any books written on this topic that are still readily available to the layman.