In a previous post, someone finally clued me in with a Scroll of Spell Extension actually did, and which spells/scrolls it worked on. Once I read that, I decided to alter somewhat my Volcano Lottery strategy.
In addition to buying/selling the book and the chest in the store for a profit, and not selling books from the first couple levels (thanks to posters on the blog) I've also started saving Time Warp and Spell Extension scrolls. Previously the Volcano Lottery was limited to the duration of two Stealth scrolls, however if you read a single Stealth (in addition to any other buffs you may have) then get 3 to 6 mobuls back in time thanks to a Time Warp, you can then read the Spell Extension scroll and double the duration of your buffs. This can also function as a good timer for your Stealth as the short-term buffs such as Haste will drop off before the others.
Even after I've gotten back from the Volcano if I've gotten a good Time Warp/Extension combo, I have gotten as low as level 5 in the dungeon before the Stealth wore off. Usually in my strategy I bank the money I've gotten from selling the Volcano books/chests and go visit the University so that money will collect interest as I buy the courses. (Only if I think the interest will get me enough money so I can read the books instead of selling them, or if I didn't find much and need the interest to get up to about 200,000 gold). BUT if I have had good Time Warping, I will not even buy my weapon/armor upgrades and will just dive into the dungeon until those buffs wear off.
Another addendum to the Volcano Lottery strategy, is that if I happen to have a scroll of Hold Monster and I can figure out where the D. Prince is sitting, I will hit him with a spell to get him to move off the potion, read the scroll, get the potion and then Shift-Z out of there. You can win the game without ever going down in the dungeon.