I was wondering if there is an equivalent to the blue book of cars for planes that can inform how much a private jet can be sold or bought for if it doesn't have any documentation (for example, no maintenance books, no logs of any kind, no manual) but it's otherwise fully operational? For example how much is the sale price reduced in percentage terms compared to a jet with full documentation?
There is no "blue book" for aircraft without documentation. Your best bet is to research government and other auction sites that focus on the disposal of repossessed or confiscated aircraft and look at their historical sale records. Here is an example of the type of website you can check. But even with this info you will only get a vague idea of value, because there can be many other factors playing into the final value of such things. This is the kind of niche market where you can easily lose a lot of money if you don't know what you are doing, or make a lot of money if you do.
The price for a private jet with absolutely no documentation is, and I'm serious here, approximately the scrap value of metal in the plane. Some parts of the plane that are not subject to time or cycle based inspection and maintenance may be of some value depending on their condition.
If no records can be compiled, and no history established, the process of getting the plane certified back into operational status is going to be such a pita, it most likely is not worth the effort.
The possibility that such a plane would emerge is quite small. Unless the plane is reduced to a heap of scrap you'll have serial numbers all over the plane, so at least a part of the history can be tracked down. The longer the period of obscurity for the plane is, the deeper in certain matter a person wishing to operate the plane in a legal manner will be.