There are baselines where books at or below it don't matter
Ars baalos already gives +6 blood boil and +5 consume
Measured wrath already gives +4 summon tainted
Ars diabolos already gives +7 apocalypse and +25% fire
The only skills you can really benefit from books with just +skills are prebuffable ones, like summon tainted, bind demon, consume, summon goatmen, blood oath, demonic mastery, etc.
But most eldritch skills, all the weapon related ones, can't spawn on tomes. No hex siphon. And tier 1-2 skills won't spawn on high ilvl tomes, so no mastery / goatman / oath.
As such really the only useful tomes you can keep are;
>+5 to certain skills that are level 30 required or less, for LLD purposes
>+6 to consume, which right now only one person is actively searching for on trading forums (and +5 is useless)
>+5-6 to summon tainted (preferably with more demonic mastery, so the ideal is +3 demon skills, +3 summon tainted)
>+4-6 to bind demon, where the baseline is a +3 base can be turned into a splendor for +1/4 to mastery/demon, so anything better than that is an upgrade
In general the fire skills are useless on tomes because diabolos exists and eldritch skills would be useless because ars meph exists but they also don't exist in the first place, and magic skills are still mostly useless because of ars meph
and even if you could go through the laborious process of cubing a low ilvl high qlvl rare tome to get +2 warlock / +3 goatmen / +3 mastery, nobody would want it because goatmen also suck