But why? How does it serve the plot? So far as players are concerned evil villain is evil. It sounds more like you are treating your antagonist as a player character in a solo game rather than trying to entertain your players.
I haven't actually started this campaign yet. I'm still in the pre-planning phase and am trying to be thorough to prevent repeating mistakes I made in my last campaign.
In my previous campaign I had intended for a water-themed lich to be the BBEG who would use a special plane-shifting fortress battleship called Krakencore to drown entire coastal cities and reanimate the drowned as undead under his command. Unfortunately, I spent so much time detailing Krakencore and the underground base it teleported to for repairs and resupplying, Darkfathom, that I neglected to figure out the lich's history, his lieutenants, events he's been involved in, and the various defenses and countermeasures he might use (I was also unfamiliar with the rules for certain spells, which probably let the wizard PC who took out the lich like a chunp get away with things he shouldn't have been able to do by RAW).
I'm taking the opposite approach with this new villain. I want to figure out everything about her that the PCs could conceivably find out. I want to have stories for her rise to power, what transpired in her quest to obtain the Book of Vile Darkness (as well as three other legendary treasures), and concrete plans for her defenses, countermeasures, tactics, and lieutenants. I want to treat her as a PC in regards to certain things to familiarize myself with the usage of spells like scrying, planar binding, plane shift, teleport, and forbiddance.
I've also decided she obtained her copy of the Book of Vile Darkness from Baltoi, a unique marilith in the Ravenloft setting who acquired it before being imprisoned in a mountain in a Domain of Dread. I've already worked out how my villain found out about and located Baltoi (it involved an enslaved diviner who has been physically mutiliated and subjected to modify memory spells so that he only knows and is able to cast Contact Other Plane on the BBEG's behalf). I'm even planning to run a battle in which the BBEG and her forces attempt to defeat the enhanced marilith (who I'm also planning to give legendary and lair actions) and the three glabrezu demons she summons so I can have details of what went down in this battle for the Book of Vile Darkness in case the PCs capture an interrogate yuan-to who was there or knew someone who was there.
As for the Book itself, the BBEG originally just wanted to glean whatever useful secret lore she could get from it, like secret treasures or true names or new spells, before putting it into a vault somewhere. However, since I rolled that the book gave her a plethora of condition immunities (charmed, stunned, petrified, etc) I decided that she is know inclined to keep it attuned to her, despite suspicions that Vecna may be using it to spy on her or spirit it away when she needs it most.