The Tome of Tranquil Mind is very useful for when you want to swap talents often, and are also pretty valuable on the auction house. With this guide you will be able to farm 20 Tome of Tranquil Mind per hour, and considering that these are usually worth 250g each on the AH, you can get 5,000 gold per hour with this method.
Some might consider this guide an exploit, so here's your warning. I don't know if Blizzard considers this an bannable exploit, but who knows how they will react.
So what you need to do is to create a Class Trial Character. These are already level 100. Log into this and use your Hearthstone to Stormwind or Orgrimmar, obviously depending on your faction. Fly to Durotar / the docks in Stormwind and talk to the NPC Holgar Stormaxe / Recruiter Lee and get a free teleport to Dalaran.
Upon getting to Dalaran you get an autoaccepted quest that you can turn in for 5 free Tome of Tranquil Mind! Send these to your main or your auction character, and delete the Class Trail Character. Now simply do this again for 5 new tomes! You can create 4 Class Trail Characters per hour, so you need to take a break every now and then, but it's definitely a really fast way to get Tome of Tranquil Minds.