Traditionally, a druid works from an oral tradition and prefers 'oral maps' carried in their heads and recited as needed. Such maps use obscure theological and astrological references (things druids knew well) adding to a druid's ability to spout gibberish like it means something. Of course it does but only other druids will know what's meant.
Rogues and other exceptional folks may puzzle it out though...
However, Tir Nan Og is a bit different from classical 'Celtic' - libraries exist in Dinas Emrys, Durrington, Glastonbury and Drunemeton - cured hides or bark scrolls and Ogham-carved wands. Getting a 'book' out means a sizable debt to a druid or Drune or theft from the same - not something undertaken lightly.
Most other places have lost knowledge to the ravages of Time barring certain tablets and ruins in the lands of the Titans or hidden in Atlantean ruins. Fomorians may prefer to use less perishable media for writing or favour an oral tradition despite their mechanical aptitudes.
Some of the more erudite nobles and educated types in Albion and the lands of the Nemere might have small libraries which look like a stack of wands or a chest full of rolled up scrolls. These should be exceptional though and note the owner as a 'cultured' individual who may be capable of battle plans and other tricky behaviour.
Rogues (and Drunes) are more likely to write things down though - maps being made by merchants and explorers in the main, those nations who use skyships would also benefit from such so the Falians and Lyonesse merchants might - as may Midgarder raiders.
Such documents wouldn't be frequent but would be valuable to those who knew what they meant - the book of plans carried by Slough Throt in Sky Chariots is of immense value yet Slaine and Ukko are indifferent to it!