Though Blizzard issued its own warning previously, but as we're still seeing players make irreversible mistakes they regret, we'd like to mention it again: be careful when using the Hearthstone Strategy Guide or Supreme Manual of Dance on your garrison followers. The Strategy Guide's tooltip reads Use: Teaches a follower the Hearthstone Pro trait. What it doesn't tell you is the Hearthstone Pro trait will replace one of your follower's current traits and you cannot choose what is replaced. Numerous players have used the item under the (reasonable) assumption that it will be added on to the current list of traits, losing something essential in the process. If you accidentally wipe out the Blacksmithing trait from a follower you intended for your forge, you might not be able to get another follower with that trait unless you build a level 2 inn and recruit one from the headhunter.
The Hearthstone Pro trait increases experience earned on follower missions by 35%, so it can be useful for leveling new followers, but be cautious where you use it. Similarly, the Dancer trait from the Supreme Manual of Dance is also useful, granting a follower the capacity to counter Danger Zones on missions. Note that Dancer is a trait and not an ability, meaning the follower who uses it will be able to counter 3 abilities rather than the typical maximum of 2. The Supreme Manual of Dance is an excellent thing to use on a follower dedicated to running missions and does not already have the ability to counter Danger Zones, but use it with care.