I've never really saw a purpose in using 'guides' I prefer just reading the words directly from the author, but guides to serve as good references and some interesting insights.
As
Morothon says, stay away from David Day...he is just a blight that mixes book stuff, with movie stuff, and throws in a bunch of his own twisted made up stuff and slaps on Tolkien's name. What he does I find absolutely disgusting.
The best one out there is probably Robert Foster's guide (as
Lal mentions). Foster even has Christopher Tolkien who admits to using it...so he must have done something right.

The good thing about Foster's guide is he gives plain, unadultered info and doesn't take great 'leaps of faith' or assumes anything. He gives the references and info for anyone who wants to go and find everything for themselves and make up their own decision (which is something I like to do). So, Foster does exactly what a guide should do, and sadly there are many many so-called 'guides' out there who don't.
I see
Mith mentions Tyler's guide...which also isn't a bad one. But it really is nothing compared to Foster's. Tyler's guide came out long before Unfinished Tales and some of his 'leaps of faith' turn out to be flat out wrong. And I find the purpose of any guide is to give good and accurate information...and thats what seperates the 'ok guides' from the 'great' ones (and the 'David Day guides').
So, I guess if you're looking for one, I suggest Robert Foster's The Complete Guide to Middle-earth...CT recommends it and uses it, so it's got to be the 'best of the best'