So I am currently working on developing a Bishop that wants to learn basically everything Priest, Psionic and Mage related. The plan is to buy most of the available lower level spells and do a reasonable (but not insane) level of grinding up the skill stats.

Through searching through previous posts I saw one that said that the ability to learn a bought spell book is a function of your Mastery skill (e.g. Mage, Alchemy, Psionics, Divine) and your Realm skills (e.g. Fire, Water, etc).

The specific relationship is your Mastery Skill plus 10% of the individual Realm skill.

It also went on to state that for 2nd level spells the threshold is a total of 15 and for 7th level spells the total is 90.

My question is does anyone know what the knowledge thresholds are for 3rd through 6th level spells?

Spell lvl 2: requires caster level 3 and casting skill of 15 (casting skill = magic school skill + 10% magic realm skill)
Sl 3: lvl5 | 30
Sl 4: lv8 | 45
Sl 5: lvl11 | 60
Sl 6: lvl14 | 75
Sl 7: lvl 18 | 90

Spells that can be cast by multiple caster schools train the higher school skill!
Based on that, for my 4 school bischops i make sure to keep wizardry skill the highest,
followed by psionic which is quite fast to grind via spamming Divine Trap on a trap,
Alchemy is trained almost entirely by mixing from skill value 15 on,
divinity imo does not really require much training in early to mid game as long as GA and MS can be cast at high PL

Find more info on spellcasting mech incl a slightly different approach for 4 school bishops here:
https://www.zimlab.com/wizardry/recovered/wolfie/default.htm

Thanks for that information. Quite useful.

Exception: If the Bishop is silenced at the time of casting, Alchemy will be trained, even if another spellbook skill that the spell is in is higher. (If the spell isn't an Alchemy spell, the character will refuse to cast the spell, making the usual muffled noises that silenced characters make when they try to speak.)

One thing I *don't* know is, what happens if spellbook skills are tied? Which skill gets the practice in that case?

All this information has been helpful in my re-re-play through of Wizardry 8.

Maybe I will actually finish it this time.