Even if not used, how dangerous is it to merely possess The Book of Solomon?

In the example you cite here with The Book of Solomon, you are referring to the dark books that were created to provide a working handbook of symbolism that can be used for nefarious purposes to comport with dark beings of all kinds, to summon them, and to link to their energies and create various events of a dark nature to happen. As such, the book does hold great danger. It is quite real that one can summon demons and task them to attack an enemy, for example. We do not like advertising their wares, but you are asking for a clear answer here about the dangers. What people who practice the dark arts fail to realize is that when they comport with evil, they are attacking their own soul and its integrity, and that will take a toll on them. It will represent a major setback to their soul’s progression and there will be severe karmic penalties that they will have to pay at some point if they indulge in dark doings of any kind. Such activities are never benign, they always cause some harm, but more harm is always caused to the perpetrator than to the victim. This is the great error made by the criminal or by anyone seeking power over another to gain an unfair advantage done for selfish purposes. Healthy and fair competition can be an incentive to maximize efficiency and overall accomplishments for the betterment of all, but when one gains an unfair advantage by exploiting the weakness of another, or cheating in some way to win a contest unfairly and cause someone else harm, when the superior outcome is not deserved or earned in a fair fashion, there will be a karmic consequence for the perpetrator of this injustice. This is always so and karma is a relentless enforcer of these truths. There is no escape other than through divine grace which also must be earned in some way. So this work holds great danger to anyone seeking to exploit its capabilities. The light holds much greater power than the darkness. To side with the darkness is a tragic error always because any short-term gains will exact a penalty manyfold greater through the workings of karma.