David Day's book A Tolkien Bestiary is an illustrated guide to creatures and characters in Tolkien's Middle-earth. It can be entertaining to read, due both to the attractive pictures and the vivid descriptions that Day weaves out of many little-known details from Tolkien's books. However, it is important to be aware that a considerable number of other details in those vivid descriptions were invented by Day himself with little or no justification in the texts, and that these extrapolations are not distinguished from the justified facts in any way.
To demonstrate this, Conrad Dunkerson has cited the following entries from the book. Exactly one of the four "reflects the texts accurately without invention", in his words; see if you can figure out which.
[Taken from an article on rec.arts.books.tolkien on 28 Sep 2000: Msg-ID: CxPA5.email@example.com]
"DWIMMERLAIK ... In the land of Rohan in the time of the Riders of the Mark, all such haunting spirits were named Dwimmerlaik. Such were the superstitions of these Rohirrim horsemen that even the Elves of Lothlorien and the Ents of Fangorn were named Dwimmerlaik and were thought to be similar evil spirits."
"FLIES OF MORDOR ... These were grey, brown and black insects; they were loud, hateful and hungry, and they were all marked, as Orcs of that land were marked, with a red eye-shape upon their backs."
"GIANTS ... Also, in the tales of Hobbits, there were rumours of great Giants who, in league with Orcs, guarded the High Passes in Rhovanion."
"TELCONTARI ... He chose Telcontar as the name of his House for this was the Quenya form of Strider, the name by which he went in his years of exile. His descendants and successors preserved the name of the House that Aragorn had founded, calling themselves the Telcontari."
You may have recognized that the "Flies of Mordor" entry was the accurate one, but the styles of these entries are virtually identical: only someone who already knew the books well would pick this up.
What's wrong with the others? Under "Dwimmerlaik", there is absolutely nothing in Tolkien's writings that suggests this term was applied to the Elves or the Ents by anyone. I know of no basis for the claim that the Rohirrim were unduly superstitious, either, and I would describe their impression of the Elves and Ents as "perilous, if they exist" rather than "evil spirits".
Under "Giants", Tolkien's writings give absolutely no indication that the Giants were "in league with Orcs" or that they were actively "guarding" the mountain passes. In fact, in The Hobbit Gandalf considers getting a giant to block up the Goblins' new gate. As for "in the tales of Hobbits", the only example I know of is The Hobbit itself; using the plural "tales" is clearly an extrapolation (whether it's a safe one or not). As for "Telcontari", it's worth noting that this word does not actually appear in Tolkien's books (though most agree that it would be the correct plural): Day's firm comments here are an extrapolation from Tolkien's texts.
In short, it is not wise to rely on this book for information on Tolkien's vision of Middle-earth. If that is not a problem for you, then those invented details can make the book a very enjoyable read. (Just don't quote it in discussions of Tolkien's actual works!)
I have not read any of Day's books all the way through, but I have read or skimmed parts of at least four of them. (I know that makes me less than well qualified to write even a limited "review" like this; I apologize for that, but with the help of Conrad and others I hope I have been fair.) In every case, my impression has been exactly as described above: they are interesting collections of information about Middle-earth, but they all tend to extrapolate from Tolkien's own writings in order to make Day's books more fun to read. Again, this isn't a problem if you want to enjoy them for their own sake, but it can make them quite deceptive as references for Tolkien's world.