According to
uo.com
My Scribe has 110 mage/eval and 110 Necro/SS
When I make Magery spell books, mods drop randomly
When I make Necro books, It does Not use Necro or Spirit speak as a mod, but will add +Magery and +Eval
Same with Mysticism books, though I don't have Myst on this toon, it made Myst books with +Mage and +Eval mods
I'll be glad to show them to you
Inscription – Ultima Online

Books with Properties
If you do have skill points in the arts you will be able to write books which have properties such as lower reagent cost, mana regeneration, skill bonus etc. To write magery books you will need magery skill, to write necromancy books, necromancy skill and to write mysticism books, mysticism skill. Likewise the + skill possible on these books will also be appropriate to the book type. While + resist spells, + evaluate intelligence and + meditation are possible on all three book types you will only get + magery skill on a magery book, + necromancy skill on an necromancy book and so on. Obviously the higher your skill, both in inscription and the book-type skill, the better the books you can potentially write.My Scribe has 110 mage/eval and 110 Necro/SS
When I make Magery spell books, mods drop randomly
When I make Necro books, It does Not use Necro or Spirit speak as a mod, but will add +Magery and +Eval
Same with Mysticism books, though I don't have Myst on this toon, it made Myst books with +Mage and +Eval mods
I'll be glad to show them to you