Woddeley's Occult Primer
The very first textbook acquired by any student wizard at the start of his first year at Unseen University. On a scale of potency, this is presumably at the very opposite end of the scale that things like the Necrotelicomnicon are at the far end of. Another beginners' book (albeit for slightly older and wiser students) is the Necrotelicomnicon Discussed for Students, with Practical Experiments.
In Jingo, we are given a glimpse of a Book Depository in the University from which an assassination attempt has (ostensibly) been made on a visiting foreign dignitary in a time of tense diplomatic confrontation. A background detail is stack upon stack of timeworn copies of Woddeley's, all of which are sorry, broken-spined, coverless, and much-abused by generations of students. Anything really potent would not have been published in such numbers, nor would they be disposed of so casually.
Certainly, when Angua picks up a dog-eared copy out of curiosity, and reads aloud from Chapter Fifteen: Elementary Necromancy ("First, you need a spade"...), she is unscathed, and totally untroubled by any of the things that might beset a non-wizard who tries to read from a really potent grimoire. There is no sign at all of her brain liquefying and trickling out of her ears, and her eyes totally fail to melt.
Woddeley's would therefore count as the ideal primer for fledgling wizards.
Indeed, in Moving Pictures, Victor Tugelbend and Ponder Stibbons are seen using the book for this very purpose as they cram for their final exams, taking in topics such as Yob Soddoth and The Infernal Star-Toad with A Million Young.
It is also very probable that in the latest edition, Chapter Fifteen has now been revised to take in the reality of changes in the nature of wizardry, as it moves away from the dribbly-candle into the bright new dawn of modern technomancy. The latest edition may read: