B was my thought—or at least I’d definitely check if I were Harry. But checking every book in the section seems time-consuming and suspicious. I think we should assume there is in fact a standard book on Memory Charms there. Doesn’t mean it contains a single truthful word.
In canon, Hermione casts Obliviate in her 7th year (presumably without consulting a restricted text from the Department of Mysteries), so the widely available book may actually have enough information for an intelligent reader to learn how to cast it.
B was my thought—or at least I’d definitely check if I were Harry. But checking every book in the section seems time-consuming and suspicious. I think we should assume there is in fact a standard book on Memory Charms there. Doesn’t mean it contains a single truthful word.
Even if truthful, it may not actually say how to cast.
In canon, Hermione casts Obliviate in her 7th year (presumably without consulting a restricted text from the Department of Mysteries), so the widely available book may actually have enough information for an intelligent reader to learn how to cast it.