If you look at the back cover, The Dune Encyclopedia may seem to have been the ultimate accessory for the fan of Frank Herbert's Dune series. What is written there makes it sound as though it contains systematically ordered material from the archive of Herbert's own background notes to the series. It lists specific items, which are mostly exaggerated descriptions of articles in the encyclopedia itself ("complete guide to the art of kanly", for example, just means a description of a few of the more common methods of assassination). It is endorsed by Herbert, so it can nevertheless, so it can lay some claim to being authoritative. (He explicitly reserves the right to change the ideas in any later books written in the series, and certainly did so: both Heretics of Dune and Chapterhouse Dune contradict the Encyclopedia.)
The Encyclopedia is not, however, a systematic collection of Herbert's background materials. I am sure that these existed (given the complexity of Herbert's imagined setting for these novels), and they may well have been used to produce much of the material here. The individual articles vary wildly in interest (the one on the geology of Arrakis is particularly yawn inducing) and quality. There is little consistency in coverage; for example, the biographical studies of the major characters are too variable in type and depth to be permitted in a real encyclopedia (which this pretends to be; it uses the conceit that it is a reference work of the far future). The one to avoid is the account of Jessica, mother of Paul Atreides, as the fulfilment of each of Jung's list of human archetypes - male as well as female - and particularly what it has to say about the archetype of "Mother".
Characters are invented for no apparent reason - a playwright, who is basically Shakespeare, right down to the details of the authorship controversy, for example. Some of the articles are distinctly ill advised - whoever wrote the Imperial Poetry account and included quotations from the best poetry of the period must have a very high opinion of their own writing.
There are interesting articles among them; these are mainly the ones to do with people from before the date of the first book, such as the early emperors, the founders of the Spacing Guild and so on.
The conclusion is that this book could certainly have done with a firmer editorial hand.