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I would say the game manual (which can be found in your AoD folder), but as that hasn't been updated at all, a significant part of it is not relevant to the current version (1.08) of AoD.

While these apply to HoI2, some of the advice works for AoD too: http://www.paradoxian.org/hoi2wiki/index.php/Assorted_Guides

You can also see the effects of terrain on combat in the Statistics tab (top right of the screen) whenever you load up a scenario, then clicking on Army/Naval/Air Unit Modifiers.
 
The rate of reorg is proportional to (1 + Morale - 0.3). For land units it is also proportional to ESE and Infra. For land units also the org/max org ratio is important:

Code:
percentage of max org | multiplier
0-25%     : 1
25%-50%   : 2/3
50%-75%   : 1/3
75%-100%  : 1/6

http://www.paradoxian.org/hoi2wiki/index.php/Reinforcements,_Reorganization_and_Bases

How losses are precisely calculated is not common knowledge. Is is known that AoD changed how this works. The higher the defence value the lower the losses, but at very high defence values the reduction of losses is small.

For landcombat there is the difference between defence and toughness. Defence is used at defence, toughness at attack. Tanks are very good at attacking but let them be attacked and they will be slaughtered. I have seen germany losing 40 armour divisions in few weeks. They were completely annihilated when still having plenty Org.

For land combat there is the difference between hard attack and soft attack. The losses both in strenght and Org can be estimated to be proportional to (hard_attack * (1-softness) + softattack * (softness)) :

http://www.paradoxian.org/hoi2wiki/index.php/Battle_mechanics_and_formulas

I assume that the estimatation is just an estimation and in truth it is that softness just says how probable it is that either soft attack or hard attack applies. If the attacker has no such attacks left, then no reduction in Org and strenght of the defender will occur.

For land combat there are battle events. The magnitude for their effects can be changed in \db\misc.txt. At the bottom of the combat section you can read their values.