The real-life Kitab al-Bulhan is an amazingly illustrated treatise on astrology and geomancy. It looks like I'd expect a spellbook to look. It depicts constellations, sinister-looking djinni summonings, and what I presume to be semi-magical fauna.
If you are going to have your wizard find a scroll or spellbook, you are honor-bound to make a handout along these lines.
If you go with spellbook, you might want to include a few extra options: so long as they hold it in their hands while casting a summoning spell, the summoned creatures have +2 attack. Perhaps you should also include a ritual whereby one can summon a perilous djinn and interrogate it? Yes, definitely.
Whispering Djinn Answer Table (d10)
If you spend 100gp on rare incenses and various metal powders, you can spend six hours to conjure an invisible spirit possessed of significant metaphysical knowledge. You attempt to coerce magical secrets out of the inherently hostile creature, with varying results (reroll redundant results excepting 1s):
- Thou Hast Erred: You summon a djinni, all right, but your binding is imperfect. The djinni is able to spontaneously combust your body, inflicting 5d6 fire damage, Will DC 16 for half. If you survive, you can hastily dismiss the creature before it repeats the effect.
- Accursed Theology: At great length and with persuasive eloquence, the djinn claims that the gods of your world are mere spirits, either misguided or malicious, and that the world is the product of a single, remote deity who does not walk the world. Unless you can somehow erase this knowledge or the authority of its source from your mind, you permanently have advantage against divine magical attacks and resistance against healing spells. Potions still work fine, though.
- The Secret Language Of Insects: Given a little time to zone in on the precise dialect, you can communicate with insects or the gestalt intelligence possessed by swarms of insects. They are completely hostile to mammalian life, but respect certain rules of propriety with regards to the exchange of gifts and services. If you displease a swarm of insects, they will have no problem at all reciting blasphemous secrets to drive you stark-raving mad, usually for days at a time.
- Ingots Of True Flame: The fires of the mortal world are ghostly shadows of the flames that dance beyond creation. An ingot of true flame is dangerous for material creatures to possess, but has its uses. A dedicated pyromancer can fashion true flames with a wood-burning kiln, 100gp of esoteric woods and spices, and a day or three of unsleeping chanting and tending. The result is a wefting knot of constantly-reshaping red metal that superheats and destroys most non-magical things that it contacts, but that can also be used to empower fire spells. Whenever a spellcaster casts a fire spell while possessing the ingot, one of the dice of fire damage is maximized and the ingot has a 1-in-6 chance of being destroyed. How one safely handles the ingot is left to your imagination.
- The Adverse Words: The beings of the material world are fragile. You learn a complicated series of otherworldly phrases that amplify entropy in whatever creature you recite them at. The targeted creature suffers 1 disintegration damage per full minute that you recite these words, or per hour if you merely think them. The creature must be within 100 feet of you and within your line of sight, though if they are within 25 feet line of sight is not necessary. The words lend themselves well to extended recitation: if necessary, you can chant these words for days at a time.
- The Elevation Of Metal: In the world beyond worlds, there exists a realm of purified matter only somewhat removed from the divine world of the gods. All alchemy may be viewed as a process attempting to purify matter and return it to a divine state. You have learned the secrets of purifying metal such that it approaches one of the earlier stages of metaphysical purity. With this process, you can spend three days and 100gp on a metal weapon or armor, imparting the "Empyrean" prefix, e.g. "Empyrean Chain Mail +1". In weapons, the empyrean prefix causes the weapon to inflict +1d6 damage against non-elemental outsiders. In armor, the empyrean prefix imparts a +1 to all saves.
- The Inverted Zodiac: Though most educated spellcasters are passingly familiar with the zodiac, there is a secret zodiac consisting of stars and other stellar bodies not visible to beings in a normal state of mind. By consuming 50gp of carefully chosen hallucinogens and invoking stellar entities for an evening, you can perceive the horrifying astral archetypes and cast an accurate horoscope for a willing ally. Roll 1d4: on a 1, the targeted player has a -1 to all d20 rolls for the next day. On a 2-4, that is how many times a player may add a 2 to a d20 roll during the next day; this can even occur retroactively to make a player succeed at a roll that they would otherwise fail.
- The Virtues Of Herbs And Beasts: Many strange beasts and plants inhabit the earth, and many stranger still beneath and beyond it. You have learned the salubrious value of several strange creatures, many of whom you are able to create as needed via spontaneous generation. Rather than brewing potions of healing the standard way -whatever that might be- you lure, conjure, and create creatures whose bodies naturally contain potent healing alkaloids. The short of it is that you are able to brew healing potions for a cost halfway between the normal purchase cost and the normal sell cost. For example, if potions of curing cost 50gp in your game and can be sold to NPCs for 25gp, you can create them for 37.5gp. The process of rapidly spontaneously generating and then slaughtering strange beasts and plants is horrifying to behold, messy, and probably heretical.
- Phylactery Against Misfortune: You inscribe potent spells against bad luck and other ill occurrence in tiny symbols on a thin scroll, wind it up, and place it within a sacramental box. The custom inks and other ingredients cost 100gp in total. When worn as an amulet, the scroll provides a +1 bonus to all saves, until you roll a natural 1 on a d20 roll or an enemy critically hits you. If that occurs, the roll is treated as a normal failure and the amulet loses its magic.
- Motes Of Unlight: You learn the secret of harvesting motes of "unlight", a poisonous, bewildering energy that negates light and confounds proprioception. These motes can be stored in a specially prepared Gloom Lamp costing 200gp. Every 1d6 days of harvesting, the lamp will be sufficiently full for use. When activated, the unlight floods a 15-foot radius around the lamp for the rest of the encounter, blinding and slowing creatures within that radius for 1 round, Fort DC 16 negates. Alternatively, the unlight may be expended to restore a semblance of life to a recently dead, relatively cohesive humanoid cadaver, lasting for approximately five minutes.