Loot can be found in Minecarts with Chests that spawn in Abandoned Mineshafts. The Loot consists of items from 3 different groups each with their own distribution: In addition to the loot found in Chests, the Mineshafts exposes the Ores of the surrounding Biome, making it a good place to mine.
http://minecraftpocketedition.wikia.com/wiki/Abandoned_Mineshaft
You have to remember mineshafts in general aren't always going to make sense. You dig a min shaft to find ore. While others keep digging that mineshaft looking for more ore you have other guys digging out the ore vein.
https://www.reddit.com/r/Minecraft/comments/xktgh/real_abandoned_mine_shaft_vs_minecraft_abandoned/
Abandoned Mineshafts are naturally Generated Structures that were added in Update 0.9.0 that form underground. The Structure can be found in any Biome, but is placed underground which makes it a little harder to spot. The Mesa Mineshafts, found in the Mesa Biome, can spawn at the surface, making the entrance more visible.
http://minecraftpocketedition.wikia.com/wiki/Abandoned_Mineshaft
Clock: this is only for the Overworld. If dawn turns to dusk, mark every area with torches so that no hostile mobs can get in your way. The clock is useless in The End, as there is no Sun or Moon. Bed: if you place a bed in a stronghold, and if you die, you're still in the stronghold.
http://minecraft.wikia.com/wiki/Stronghold
It is possible and relatively common for Underground Ravines, Abandoned Mine Shafts, and Dungeons to intersect a Stronghold. It is also possible for Stronghold hallways to extend out into a body of water, allowing for easier detection of the Stronghold.
http://minecraft.wikia.com/wiki/Stronghold
Some strongholds are not perfectly generated. There are only 3 strongholds in a world, if I remember correctly. If you found all of them and none had a library, you won't find a library at all. If you only need bookshelves I'd recommend you to find a village. The librarian has a house with a few bookshelves in it. To your question in the comment: Abandoned mineshafts are not only found at strongholds.
https://gaming.stackexchange.com/questions/155256/how-do-you-find-stronghold-libraries-in-minecraft
Up to 128 strongholds spawn per map on the PC edition, as of The Combat Update (vs only three on the Xbox and PS3 editions). Most strongholds have a Monster Spawners (for Silverfish), an abundance of Stone Bricks, Torches, Chests with randomized loot, Stone Slabs, and blocks found mainly underground such as Cobblestone.
http://minecraft.wikia.com/wiki/Stronghold
Mineshafts are rarely in an actual straight line, but in loops and variances. I am not saying abandoned mines are completely accurate, but they are not as far off as some people imagine. Sure, but a minecraft mineshaft is about ten times the size of that. That certainly has a layout and an order to it.
https://www.reddit.com/r/Minecraft/comments/xktgh/real_abandoned_mine_shaft_vs_minecraft_abandoned/
Abandoned mineshafts are underground structures, which are randomly generated in the Overworld. They are enormous and their structure makes it very easy to get lost in them. This includes long corridors, different height levels and many identical looking paths.
http://www.minecraftguides.org/abandoned-mineshafts/
Like dungeon chests, chests in abandoned mineshafts only contain a few items and each item has a certain chance of spawning inside the chest, as follows: 1  Bread: 20% chance of getting at least 1 loaf of bread per item stack. 2  Coal: 13% chance of getting at least 1 loaf of bread per item stack.
http://www.minecraftguides.org/abandoned-mineshafts/
where is there always mineshafts in minecraft
In minecraft, anbandoned mineshafts are randomly generated in the overworld. In minecraft, Abandoned mineshafts are underground structures, which are randomly generated in the Overworld. 