Help request with story point (LMOP) *Spoilers*

[Cerenity]

** PLOT SPOILERS FOR LOST MINE OF PHANDELVER BELOW **
Hey fellow dnd'ers, I was wondering if some of you could give me some help or advice on how to progress a story point within my current game of dnd. A little backstory we are currently all new players going through the lost mine of phandelver set, one of the players (wizard) took great interest in a particular side quest to find information about a lost spell book within the story. Because she was so interested in this plot I changed the story so that instead of the banshee in the story just giving so random far off information of its location for her to take back to complete the quest I had her explain it to be in thundertree.
Needless to say she told the quest giver she was unsuccessful in gaining the information of its location and as a party set off to retrieve said book which is of necromancer origin. After a session within the thundertree setting she and the party managed to steal the book off of the cultists in the area and escape with the book which she could feel humming with power.
Towards the end of the session she felt compelled to open the steel casket the cultists had locked the book up in fear of its power and touched the book within.....I ended the session there and left her in suspense as the conclusion (had her roll a cons save, which she failed!)
So to follow I want her to have an interesting plot point to arise that surrounds the book what happens to her and her choices in next session but I dont want it to shadow or take over the main plot of the story which is to find a black spider hiding out within the mine of power. If anyone could help me with ideas of a conclusion of sorts to this plot hook I would be most grateful.
The party are currently in the wilds outside thundertree, the party backed off away from the wizard as she opened the chest and touched the book. I originally planned for her to be able to learn a few necromancy spells from the book and have it talk to her in whispers whilst she carried it but I felt that is a little weak.
Many thanks for any help / ideas offered!

[Armored Walrus]

So to follow I want her to have an interesting plot point to arise that surrounds the book what happens to her and her choices in next session but I dont want it to shadow or take over the main plot of the story

I originally planned for her to be able to learn a few necromancy spells from the book and have it talk to her in whispers whilst she carried it but I felt that is a little weak.

If you don't want it to overshadow the module then that's not weak at all. In fact, if I wanted something to be large and campaign-changing, this is how I would first introduce it. Once you complete the module, this book might be a plot point that can slowly grow in importance. But for now, it's just an interesting side story. Come up with a cool description of it, give the player some minor bonus, and move on.

As a module, there was a reason they made the book itself far away and difficult to obtain - so that it wouldn't side track your players from the main story - don't make the same mistake (I only call it a mistake because you say you don't want to overshadow the story of the module) again by making the results of that side quest larger than the main story again. (Unless that's the campaign you want to play and you're ready to ditch the module)

Later the book can become more dangerous, or it can have more power that can be unlocked (with its own side-quests), she can find a page she previously overlooked that raises some mystery, the quest-giver can discover that your party mage lied to her, the banshee can find out the party misused information she provided, the original owner of the book can come looking for it, etc.

[LuminousWarrior]

If you don't want it to overshadow the module then that's not weak at all. In fact, if I wanted something to be large and campaign-changing, this is how I would first introduce it. Once you complete the module, this book might be a plot point that can slowly grow in importance. But for now, it's just an interesting side story. Come up with a cool description of it, give the player some minor bonus, and move on.

Lord of the Rings did the same thing with the One Ring. When it was first introduced in The Hobbit it was just a useful magic item that gave the wearer invisibility. It wasn't until LotR that the full lore and power of the One Ring came into view. I would say that this would be your best bet for an interesting magic item, and it'll sow the seeds for your next campaign.