Hm, a reliable guide to Middle-earth. David Day's works will definitely not be on the list, take our word for it on this one and steer clear of anything by Day, which should be listed in the fan fiction section. The only one that comes to mind is
The Complete Guide to Middle-earth, as previously mentioned. I've found Hammond/Scull's
The J.R.R. Tolkien Companion and Guide to be superb, but as it focuses more on real-world entries, it may not be exactly what you're looking for. However none of the published books even come close to some of the online encyclopedias which are available. Below are some of my favorites in order of most-used to least-used.
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Tolkien Gateway
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Wikipedia's Tolkien section
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TuckBorough.net
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Encyclopedia of Arda
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Annals of Arda
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lotr.wikia.com
If you're looking for maps however, then the Atlas of Middle-earth is definitely recommended. While it may contain some inconsistencies, you really have to take a few liberties when creating a map. Images of maps are much more difficult to find online as the Estate goes after them rigorously.