From my research, a spirit/entity can be a familiar, what else would it be? As far as the deceased, no. It would be extremely disrespectful to just "summon" a spirit to do your bidding. Through my experiences of working in graveyards with these spirits, its proper to communicate with them and build some sort of relationship with them before asking them for favors. To just "summon" them, or to just ask them outright, would be like a stranger knocking on your door, pulling you out of your house, and then requiring them to do what ever they ask. Its rude. (if you get my drift here.)
Plus, some sort of payment should always be presented to them. Whether it be dimes, or something they enjoyed in life. (cigars, wine, favorite flowers, etc.)Some references will say that most spirits who are willing to help, will only want to help for one working. Others will say as many as they are willing. I would suggest getting to know said spirit. But keep in mind that they're people as well.
Back to the familiar subject, this isn't a "master servant" relationship either. Its actually quite mutual. They can actually even get rebellious at some point while working with them. You're a team working together. You need to give it some sort of energy in order for it to carry out your tasks in the astral realm, and have a strong relationship foundation. Though, familiars can be released as well, using proper methods.
I have some methods I've found quite effective on the subject if you wish to continue to matter. Just message me.