Most of the books in his library would be the bound workbooks of dead-and-gone arcanists: their records of their spell experimentations, with diagrams, notes as to the efficacy of various material components and their combinations, and the finished spells with notes on casting particulars, warnings, and reminders. These are generally small, slim tomes (akin to a modern "trade paperback" of 100 pages or less), bound in preservation-enchanted hides that have been dyed maroon, very dark green, navy blue, or deep bronze colour before being "water-sealed" by the enchantments, and are usually named things like "Arkoon's Librum" (to Netherese, a "librum" means a spell workbook) or "The Third Librum of Nemurressor." These are unique, irreplaceable tomes.
There are also five "standard" works (the equivalents of bestsellers or course textbooks) that every library will have:
Haerro's Veltanabula: Being The Triumphal Battle Spells of Vultivar Haerro, Monsterslayer, which is just what its title suggests. Full of early versions of forcecage, wall of force, cloud of daggers (moveable), mindworm spells (that lash back at illithids who've thrust tentacles into your brain), various tentacle-melting ray spells, and spells that shoot cones of beads of force like live grenades.
Sarturl's Study of Magical Fields, a dry-as-dust, exhaustive examination and comparison of various volume-filling magical effects. Sarturl was THOROUGH. Want to know what the active area of an antimagic field smells like? Sarturl's there for you, for an entire precise-of-detail page.
Oloam's On Serpentine Matters, an examination of enchantments that confer reptilian forms (or specific features, such as scales and nictating membranes) on human casters, how to vary and tweak them, warnings and pitfalls associated with them, and Oloam's experimentations on improving and combining them; his chief aim was to retain human hands and speech to keep unfettered access to "spellcasting norms" while in serpent form. His secondary aim was to make assumed serpent forms as powerful as possible (precise prehensile control, crushing strength for coils, and puissant venom, often with skin-contact delivery so biting wouldn't be necessary).
Skouloond's Spell Studies is an exploration of the particulars of verbal, somatic, and material components, and varying them in combination to alter known spells, and to craft new ones. It remains very useful to this day, for the same reasons a "substitution guide" is invaluable to a novice chef (if you don't have any parsnips, or any salt, here's what you can use instead).
Ivrynxra's Exploration of Enchantments is a chatty, funny first-person account of unusual castings, in which the lusty, jovial adventuress-mage Ivrynxra Hoond relates what happened when she cast this spell on an invisible beholder while falling in the heart of a waterfall, or when she sliced a ghost with a conjuration intended for something else entirely (yet it worked!). Her writings even discuss a neglected but vital field of magecraft: what actually happens when hostile spells intersect, or affect the same area, or actively clash.
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