Precious Mettles: Why Mettle is the best Element of the Soul and you should always pick Mettle and also avoid merging it [hence Mettles not Mettle]
tl;dr
Mettle cannot be Maladied. Use Mettle whenever there is a risk of an Element of the Soul (EotS) getting Maladied.
The best Origin in the game for newbies is The Revolutionary, because you can very quickly get 2-3 Mettle. You get Mettle as one of your starting stats, Mettle for cataloguing your Journal, and can get another optional Mettle from attuning Edicts Martial with Horomachistry. This means you can catalog 2-3 books at a time without worrying about maladies.
You should always try to get a Mettle EotS ASAP and exclusively use Mettle when it comes to activities where there is the potential of an EotS getting Maladied.
Given that Mettle’s elements (Forge, Edge) are shared by two other EotS (Shapt has Forge, Ereb has Edge,) it makes sense not to upgrade the Mettle cards and instead, keep them at level 1 so you have extra actions with which to do dangerous activities such as cataloguing books.
l;dr
You can get Mettle as a starting EotS during ‘character creation.’
Chor + Mettle = The Executioner
Fet + Mettle = The Revolutionary
Shapt + Mettle = The Archeologist*
*the Archaeologist can also be chosen if you pick Shapt + Wist. The Artist is selected by selecting any EotS plus Ereb. All other Origins have a single possible combination of EotS.
Horomachistry and Illumination are the two Wisdoms that can potentially reward you with Mettle as the atunement reward for a skill.
If you do not select Mettle during The Beginning, you can still get it easily by committing your starting skills if you play as any librarian other than The Symurgist or The Cartographer
The Artist: One of their starting skills is Desires & Dissolutions, which can be dedicated to Horomachistry for Mettle or The Boss for Ereb
The Symurgist: Neither of their starting skills can be attuned for Mettle.
The Magnate: One of their starting skills is Sacra Limiae, which can be dedicated to Horomachistry for Mettle or Hushery for Trust.
The Prodigal: One of their starting skills is Pyroglyphics, which can be dedicated to Illumination for Mettle or Ithastry for Wist. Their other starting skill, Bells & Brazieries, can be dedicated to Horomachistry for Mettle or Ithastry for Wist.
The Cartographer: Neither of their starting skills can be attuned for Mettle.
The Twice-Born: One of their starting skills is Glaziery & Lightsmithing, which can be attuned to Illumination for Mettle or Ithastry for Post.
The Book of Hours wiki has a list of all skills that give Mettle based on their Wisdom atunement.
Horomachistry: Bells & Brazieries, Deep Mandaic, Desires & Dissolutions, Drums & Dances, Edicts Inviolable, Edicts Martial, Inks of Containment, Inks of Power, Sacra Limiae.
Illumination: Applebright Euphonies, Disciplines of the Hammer, Glaziery & Lightsmithing, Purifications & Exaltations, Pyroglyphics, Transformations & Liberations.
