Besides spells, I mean.
Most of us have seen college notebooks which, while in theory should be dedicated to notes and information regarding the class in question, in reality will usually have all sorts of doodles, unrelated notes, and various other inanities written in its pages. An apprentice wizard is very much the equivalent of such a student, so their 'notebook' should probably be similar in that regard. Also, once a wizard becomes an adventurer, there may be a legitimate need for such behavior - some information will need to be properly recorded, and sometimes a spellbook is the only means available to do so.
And so, when your PC adventurers stumble across a spellbook, roll 1d4-1 times on the table below:
Spellbook Extras Table (d10)
6: Spell Commentary
7: Magic Item Commentary
8: Monster Commentary
9: Religious Notes
10: Roll twice, combining the results
Art - roll a d6 on the table below:
1: Portrait(s) done in a somewhat realistic style (could be of anyone - the apprentice's master, family and friends, fellow adventurers, the bar wench the wizard encountered the night before, etc)
2: As above, but drawn in a more erotic/lewd manner
3. As above, but drawn more for humorous/mocking effect
4: Non-realistic doodles and/or geometric shapes
5: Portrait(s) of monster(s)
6: Portrait(s) of magic item(s)
Maps - roll a d4 on the table below:
1: Partial dungeon map
2: Overland local map leading to dungeon
3: Combo overland/dungeon map
4: Map of hidden room(s) inside house/temple/castle/etc.
Poetry - there is a 5-in-6 chance that this is pretty doggerel stuff, at best being mildly amusing limericks ("there once was an elf from Hamrick..."). If the poetry is of the non-suck variety, there is a further 5-in-6 chance that it is not original to the wizard in question, but rather copied from another source, and therefore perhaps has potential adventure-useful info contained within its verses.
Rumor - a rumor/adventure seed, noted for posterity. If you have a rumor table/chart for the area, roll on that and dress it up a little. There is a 1-in-6 chance that the rumor is 'reversed', making a true rumor false and vice versa.
Spell Commentary - roll a d6 on the table below:
1-4: basic notes and anecdotes regarding the spell in question ("why did master Garrett teach me this spell? It's useless!") ("I love the smell of fireballs in the dungeon")
5: notes regarding an unusual use and/or tactical application of a particular spell
6: notes and theory regarding creating an improved version of a given spell. If spell research is followed along these lines, it will give a small bonus to the success chance of creating such a spell, although there is a 2-in-6 chance that the notes/theory are in error, in which case an equivalent penalty is assigned to the spell research roll.
Magic Item Commentary - roll a d4 on the table below:
1. Listing of a command word for a particular magic item
2-3. A listing of various items needed for the creation of a particular magic item.* If item creation research is followed along these lines, it will give a small
bonus to the success chance of creating such a magic item, although there is a
2-in-6 chance that the notes/theory are in error, in which case an
equivalent penalty is assigned to the item creation roll.
4. Description of a non-standard/unique magic item.
Monster Commentary - roll a d8 on the table below:
1-5: Notes on a monster or monsters common to the area ("the orcs of Tharn are not as primitive as we were led to believe...")
6-7. Notes on a monster or monsters not common to the local area, or are otherwise rare and unique
8. Notes on a monster not in whatever Monster Manual you're using (make something up!)
Religious Notes - there is a 4-in-6 chance that this is info regarding some demonic or otherwise malevolent order/cult that the wizard has encountered, otherwise it gives details about a minor or secretive religious order about which little is generally known.
*Appendix J in the 1st ed DMG is quite useful here, as is the 'Reputed Magical Properties of Gems' chart on pages 26-27 of the same volume.