I have always liked dipping into encyclopaedias, jumping between entries, letting the links take me to places I wouldn't access directly.
However, I do not find the thought of a work of fiction starting with an encyclopaedia that welcoming a prospect. The place for this sort of thing is (much nearere) the back of the book, where those who dislike this sort of thing** can safely ignore it, while people like me dip into it during and after reading the narrative section(s) of the work.
If, however, you mean a short extract from your fictiona encyclopaedial, that may be another matter; but even with this, you would have to be careful not to simply create an info dump that turns prospective readers off.
Current advice is, I believe, to jumpr straight into the action. (Like all such advice, there are those for and against it.)
** - These may or may not be the same people who dislike maps.
I have thought to place the encyclopedia at the end of the book. But the first book of my series is not so much about action. Its more about feeling into the pace of that world, which I think is necessary since that world is so very different from most other fantasy worlds. Its a much more organised society, its several civilisations, its conventions and social rules strange to us (profession is so important that a person is not working as a baker for example, but
is a baker, whether he is baking on the moment or not).
And its a world with industrial-level technology (not steampunk, but rather some otherworldly version of the 1950's), and technology is constantly developing.
A typical fantasy world does'nt need a prologue, since most fantasy worlds are just variations of medieval England. The reason why a lot of authors have chosen to include a prologue is often more due to kitsch than to any need for a prologue.
I thought I needed a prologue, so I wrote one in the form of a excerpt of a school textbook for nine graders there (complete with questions directed at the schoolkids at the end of every part of it). The problem with the prologue was that to explain what happened in 7778, I had to explain what happened in 7777, and then 7768, and then 7765, and then 7757. Twenty-one years might not look as too much in fantasy timelines, but medieval worlds have slower timelines than modern ones.
A reader don't need to read the encyclopedia to understand the world, but she may like to go to the end of the book and look into it. Its not a full encyclopedia, just explanation of some in-world terms, some countries, cultural traits, history and so, as orientation.