[sirinsidiator]

I made a small workaround for the next version of AGS.
It checks against the achievement:

local MERCENARY_ACHIEVEMENT_ID = 1348
local MERCENARY_BOOK_ITEM_ID = 64715
local MERCENARY_CHAPTER_START_ID = 64716
local MERCENARY_CHAPTER_END_ID = 64729
local function IsMercenaryMotifKnown(link)
    local itemId = select(3, zo_strsplit(":", link))
    itemId = tonumber(itemId)
    if(itemId < MERCENARY_BOOK_ITEM_ID or itemId > MERCENARY_CHAPTER_END_ID) then return false end
    if(IsAchievementComplete(MERCENARY_ACHIEVEMENT_ID)) then return true end
    if(itemId ~= MERCENARY_BOOK_ITEM_ID) then
    local index = itemId - MERCENARY_CHAPTER_START_ID + 1
    local _, numCompleted, numRequired = GetAchievementCriterion(MERCENARY_ACHIEVEMENT_ID, index)
        return numCompleted == numRequired
    end
    return false
end

I then call it like this:

local isKnown = IsItemLinkBookKnown(itemLink) or IsMercenaryMotifKnown(itemLink)