Let's get one thing straight. Grimoires are not Books of Shadow as we keep today. They are not recipe books for "brewing" Magick spells. The most foreboding aspect of the Occult is held within these pages; the act of Spirit evocation. Are you willing to call these energies into physical manifestation? or are you like the many who light candles and invoke unseen energies placing faith alone in your workings? These books are held in high regards in Occult circle most notably the Lesser and Greater Keys of Solomon. Most were written during the medieval period some taken from older sources and Latinized into then modern understanding. The Lesser and Greater Keys, for example, were not likely written by Solomon but taken from another source. The original text would be very different if it could be compared to our modern version. These books are meant to be used to evoke the spirit. In most cases these spirits are often of an ungodly nature and goes beyond religious teaching. Even high officials in the Church have been rumored to use such devices when their God has failed in prayer. Again, many of the spirits are taken from older sources and some are more recognizable than others belong to ancient Pagan cultures.
The Church has had a major influence over these books since many of then contain prayers relating to Jesus or Saints in order for protection. Hebrew names of God are always found within these pages of a Kabalistic nature. Many of these books were claimed by the Hermetic Order of the Golden Dawn and translated into modern versions which would later become the foundation for the Occult beliefs we have today ranging from Wicca to Satanism. Do these books work? Yes. There has been so much focus and energy placed into the belief of these Spirit beings that they can be evoked with enough time and practice.
Keep in mind that these Spirits, when first written down, were among the belief system of the Medieval period heavily influenced by the church. In my own workings I put aside modern ideas and work from my own system in order to make the energies move. It takes a great deal of practice, trial and error, and patience in order to work out the ideas written in those great works. It took me a while to realize that nothing was pure and in it's original form. Today most people will use rituals invented by the Golden Dawn in order to evoke these beings to physical manifestation. When these books were first written such teachings didn't exist so how different were the original ideas compared to what we have now? I've worked with Golden Dawn energies but I felt that it hampered my spiritual growth and created an imbalance. It wasn't for me.