I'm pretty new to D&D as a whole and a new DM who's running Lost Mines of Phandelver for some friends who wanted to roll their own characters. As a result, some of the magic items detailed within the mines don't really seem exciting for my current party and I'm trying to make some replacements to make finding them a bit more fun.
One idea I had was to leave my party's monk a scroll from an old master monk that would grant some ability upon being read (in-world kind of shown as the teachings of that master on the first steps to enlightenment). This idea mostly came from the flavor of monks caring little for material wealth (and thus likely not being interested in a general +1 monk weapon, but rather being interested in beginning a journey of enlightenment. However, when trying to either find an item whose effect kind of worked for that (Referencing the Player's Manual and Lost Mines Book as a starting point), I found that items that just impart knowledge/training/improvements to class abilities seem exceedingly rare in D&D 5e. Are items like these avoided for balance reasons or more for the general flavor of "magic items"?
I'm hoping to create an item with similar power level to Hew/Lightbringer which are +1 uncommon weapons with conditional side effects against specific enemy types. However, I also want to make sure that I don't make something inherently broken in the format of D&D which will make planning any future encounters for this character significantly harder (or just make them far stronger than they should be). I'm fine with this item in question not necessary just resulting in a raw bonus to damage dealing potential and being focused on other traits instead (The skill being granted having some combat potential does feel consistent with the items it will be replacing, even if that bonus ends up being something such as a bonus to movement or empowering Ki usage in some way rather than the raw boost normal magical items have)
In summary, are items that just grant skills (whether these skills be proficiencies, or combat boosting effects) by being consumed/read generally seen as something to be avoided in D&D? If these types of items generally aren't aimed at being avoided, what are some reference items that could be pointed out to understand how to scale these items appropriately and roughly match the power level of various rarities of items? If items that permanently teach PCs new abilities are generally avoided, what are some good alternatives to model a system for a PC beginning a journey of learning/spiritual enlightenment in an item?
Best Answer
Skill proficiencies are primarily the domain of character creation
Items that grant additional proficiencies are indeed rather few and far between. Any item that grants proficiency with a weapon, tool, language, or skill generally only does so while the items is equipped and/or attuned. For example, the Bracers of Archery grant proficiency with the longbow and shortbow, but only while they are attuned and worn. As far as I know there is no published item that allows a character to gain a proficiency permanently. The closest thing I can think of such such an item would be items such as the Manual of Quickness of Action. These items allow the user to gain a permanent increase to one of their 6 game statistics, but they are very rare.
The reason for this, I believe, is that skill and weapon proficiencies are primarily the product of character creation and advancement. That is, they are mainly granted by racial features, class features, and backgrounds, and they represent aspects of the character's origin story and personal development. Having magic items that grant permanent proficiencies entirely unrelated to the character takes away from this aspect of character creation, and might make it feel like you're taking away players' agency over their characters.
Similarly, you probably don't want to have an item that just grants additional class features (e.g. more ki points for a monk, additional fighting styles for a fighter, etc.). If you want to grant more class features, there's already an easy way to do that: level the characters up!
Consider making the scroll a part of regular character advancement/training
That being said, the above suggests an obvious way to make your scroll idea work: make it work with the normal character advancement paths instead of working separately from them. There are a few ways to implement this mechanically. The simplest is to have no mechanics at all: just state that the scroll will take time to decipher, and when the monk levels up, tell them that their new class features are partly the result of their efforts deciphering the meanings of the scroll. Of course, you've said that you want to give your character a mechanical benefit. If you are playing with XP, you could allow the monk to gain additional XP by reading and deciphering the scroll, although this might feel unfair to the other players unless you plan to provide all of them with similar benefits.
Another possibility is to allow the scroll to serve as an "instructor" to teach the monk a new skill using the rules for downtime training given in Xanathar's Guide to Everything (emphasis added):
Essentially, the scroll would substitute for the instructor, allowing the monk to skip having to find an instructor and start training as soon as they have the downtime available to do so. This way, the new skill learned becomes part of the monk's character advancement and story rather than a seemingly arbitrary boon handed down by the DM, even more so if you allow them a choice of which skill to learn. Of course, this will require you to buy XGtE and maybe also the DMG, since you've said you currently only have PHB and LMoP.
If you don't mind spending some real money for yet another source book, another potential benefit you could grant your monk player from the scroll would be learning some of the optional class features introduced in Tasha's Cauldron of Everything. These would be presented as "lost teachings" that they learn from the scroll.