That's the sixth book, "Harry Potter and the Half-Blood Prince". But if I recall correctly, the original owner of the book, the Half-Blood Prince, was none other than Snape, not Voldemort.

God I thought that was such a weird book. It attempts to build loads of tension around the idea that an old textbook has gasp prewritten notes in it. I know that it tries to imply it might be Voldemort's textbook but even if it was... would that be a huge deal unless it had something useful/interesting specifically about Voldemort? For me at least that part of it fell spectacularly flat

Actually Harry dismisses the idea about it being Voldemort very early in the book because Voldemort would never call himself half-blood.

Yeah that is true, and I do kind of remember that. Admittedly it has been a while and I have a (possibly incorrect) memory that it took annoyingly long for him to do that. My memory is also that despite dismissing it as Voldemort this text book was still treated as super ominous even after

Did you read the second book? Voldemort's diary plays a huge part in that one, so it's not surprising that the characters would find Voldemort's text book interesting or ominous (possible horcrux too).

I did read the second book but the problem is that from a story perspective another old book being a horcrux would be... kinda lame IMO

All the HP books were like that.

In most cases, somebody just doing the obvious thing at the beginning of the book would have left nothing to write about in the rest. So, they just didn't do that thing.

I stand corrected. Thanks.