Protection will stack on different armor pieces, and provides comprehensive protection against all forms of damage except The Void and server kill commands. It is true that falling, fire, lava, poison, and even mob damage will be reduced, but false that the armor durability will always reduce more than normal. When falling off a building with Protection IV boots, the boots will not suffer any damage, producing the same results as feather falling III boots. The Armor will take more damage from Mobs and Players however.
It is possible to get feather falling IV AND protection IV boots, reducing fall damage immensely. All other pieces of armor with protection will add additional damage reduction in the same way the boots would. Protection appears to be one step down from specific defenses (ex: Protection III = Feather Falling II when taking fall damage, or Protection III = Fire Protection II when in lava).
TL;DR- Protection enchantments are slightly less effective than specific ones, but protect against everything at once. The best possible armor would be:
Helmet: Respiration III, Aqua Affinity I, Protection IV, Thorns III
Chest and legs: Protection IV, Thorns III
Boots: Protection IV, Feather Falling IV, Thorns III
Note that Only highest Level of Thorns take effect if present on multiple peices of armor. Source
What is an enchanted book?
According to the Minecraft Wiki, an enchanted book is an item which allows players to add enchantments to weapons, tools, or armor by combining the enchantment book with one of said items on an anvil.
Can they receive any enchantment?
Testing in creative and survival mode shows that books can receive any enchantment that a normal weapon/armor/tool/other can receive. An easy way to check is to go into creative mode and use the new search function to search for the term book. This will show you what all the enchanted books are.
Can they receive more than one enchantment?
According to the wiki, books may only have one enchantment. This can be tested by giving yourself a bunch of books and enchanting them, then throwing them into lava or whatever you wish to do with them.
Since enchantments have different chances of occurring for different tools, materials, and at different levels, how are those probabilities different for books?
This would be different for books since books are not diamond swords. However, this would have that many variables because unlike a sword, books can only be made from paper. Swords can be made from diamonds, iron, and other materials. You would have to test this to truly know, but if John tells us how many levels he invested into his enchanted books, we might be able to know. Right now, though, it appears that the enchantment gives you the highest level (ex. Protection IV, Sharpness IV, etc.)
Best Answer
The Redpower2 mod for Minecraft removes the default chance of a mob dropping its head, and instead adds the Vorpal enchantment. When killed with a Vorpal enchanted weapon, mobs and players have a chance to drop their head, increasing with a higher Vorpal level. Unfortunately, this affect does not work on wither skeletons. The Vorpal enchantment also gives a chance for your attack to deal a large amount of damage.