Okay so what you want is to do something like the summoner background which has this
<GameModifier>
<ModType>Player</ModType>
<Attribute>AbilityBonus</Attribute>
<StrVal>A_SummonedBonusLevels</StrVal>
<Value>2</Value>
<Provides>+2 level for Summons, unlocks the Summon Shadow Warg spell</Provides>
</GameModifier>
but you can't put those types of modifiers on items. They simply don't work.
Your first option is to create a readable book like Pariden has for learning champions new spellbooks. You can do that, but instead put the above modifier in the trait they learn. This makes the item consumable, and gives the user a trait which boosts spell summons.
The other way is to provide a custom unitstats for all your items. Say UnitStat_DDD_SummonLevel. You then give it to various items of your choosing. Then you go and modify all the spells that summon units and add your new unitstat to the calculation of the summon level. This makes the summon dependent on who summons it since unitstats are individual. But it also gives you the opportunity to put extra summon-levels on items.
