Reliability of information in Tolkien’s Ring by David Day?

No it's not as a reliable scholarly source. If you like the artwork included, that would be the only reason to get it IMO.

All David Day books tend to mix speculation with canon without clearly separating them, state uncertain theories as facts, introduce ideas not actually supported by Tolkien’s texts, and occasionally make outright factual errors.

Not sure why anyone would waste their time reading a David Day book, when they could pick up Tom Shippey or John Garth, etc...

Many thanks

I think it was garm who told me many years ago that David Day books are for looking good on the coffee table but not for reading, look at the pictures, drink your coffee, and don't pay any attention to what's written in them.

I agree, though one thing I will say about David Day, is that he has used some fantastic illustrators in his books, and I am always impressed with the illustrations, apart from the horrible lung map.

Oh yes, the "lung map" is horrid.

I have also gotten the impression that he is considered, in part, unreliable. I have no David Day books and I have no plans to get any of them.

The Tolkien Society does not recommend any of Day's books in their suggested readings (preferring Robert Foster's The Complete Guide to Middle-earth instead)[4] whilst David Bratman, editor of the Tolkien Studies journal, makes the same suggestion that David Day's books are "Not Recommended".[5] Troels Forchammer noted in his blog that "Day is infamous in Tolkien circles for his creative re-interpretation of Tolkien's work"[6] whilst Michael Martinez made the sterner observation that "In Tolkien scholarship the worst insult one could deliver at any point for many years was equivalent to 'That sounds like something David Day wrote'."[7]

Tolkien Meta-FAQ author Steuard Jensen said about A Tolkien Bestiary that "it is not wise to rely on this book for information on Tolkien's vision of Middle-earth" and that "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".[8] In particular, A Tolkien Bestiary (and its derivative Tolkien: The Illustrated Encyclopedia) has been specifically criticised for its entries on Beren, Giants, long-worms, Sauron, Telcontari.

https://tolkiengateway.net/wiki/David_Day