Just tell them they're suffering ability drain.
Unless the module specifically calls for them to be unaware of the effects, you normally notify a player when their character suffers damage or drain. The first character flips through the book and goes "Whoa, wait. That's hurting my head. Ow. Don't read it, friends." Hopefully, they listen and that's that, unless they decide to keep the book and put it with some other books and trigger its "blending in" effect and end up picking it up again. Assuming they're not high enough level that restoration is trivial, that drain is going to be there for a long time anyway.
If your Vacuous Grimoire is different: The Pathfinder version of the Vacuous Grimoire is described as only "mildly interesting" and looks like a normal volume. The reason PCs pick it up and read it isn't a compulsion, it's because they swept the bookcase with detect magic and holy crap a magic book. It has a decent chance of hitting low level characters (and mid-level characters with poor Will saves) for Int, Wis, and Cha drain, but doesn't normally continue to do so, unless they cause it to blend in and pick it up and read it again.
If the module specifically calls it out as a compulsion to read the book every day, it's extra-important you inform the player[s] of their character's drastic mental changes. The player should roleplay the change in personality, and telling them about the mechanical changes is necessary so they know to do so. Charisma drain is probably easiest to work into description - the character becomes irritable and irritating to be around, snapping at other party members and generally being a pain anytime their nose isn't buried in the book. Wisdom can be represented by becoming oblivious and making snap decisions - not noticing basic details of their surroundings, reacting emotionally instead of calmly or rationally. Intelligence can be hard to roleplay, but describing a bookish character as no longer caring about other books and not wanting to solve problems, and maybe simplifying their vocabulary can work.
If the module says to keep the attribute changes secret from the players, that's a whole other issue. I guess make their Will saves in secret, and make a note for yourself to adjust all their rolls if they lose a modifier point, and hope they don't lose a modifier from Intelligence since it requires them to lose skill points, which is hard to hide from the player - which points do you take off? Since the book affects all three casting stats, there's also a chance of losing spells per day or even usable levels of spells, which is going to make things obvious quickly.
If the module calls for both the attribute drain to be secret and a compulsion to keep reading the book every day... well, that PC is probably soon to be an NPC. They're going to lose a point of Will save every time the Wisdom save fails, and be more likely to fail it again the next day. Any casting ability they had will diminish and then vanish. Include descriptions for the player of how they scream at their teammates, trip over obvious obstacles (maybe because their nose is buried in a book?), and can't remember the contents of any of the books they've read - not even the one they just read again that morning. Hopefully they or a party member will figure it out before an ability hits zero and takes the character out of action permanently (or at least, until a greater restoration can be had, which is a higher-level spell than even raise dead - that's right, the character would be better off dying than re-reading that book every day).
