Since there are SO many books out there about Wagner...
By a long shot the most written about composer in western music. The sheer amount of books is bewildering.
As a former Wagner fanatic (in my early 20's), I've plowed through a fair amount of the literature. Here are a few highlites:
The best overall introduction to the operas is still Ernest Newman's 'Wagner Nights', published stateside as 'The Wagner Operas' and available on Amazon.
It's a thick volume, but highly rewarding to study (or dip in to) as it gives great background and a blow by blow guide to the music, introducing all the major leitmotivs and how they interact and develop. I could never have managed without it.
Bryan Magee's short book 'Aspects of Wagner' is a terrific read, each chapter discussing one particular 'aspect' of Wagner. Magee does not idolize the man, and certain controversial subjects (yes, that
one) are gratifyingly met head-on.
I've lost count of the Wagner bios I have read, but there are several (somewhat overlapping) books by Robert Gutman which I can recommend. His earlier bio (possibly OOP) contains some rather uncomfortable stretches, so be warned. At one point I almost had to put the book down; Wagner was not always a 'nice guy'.
Finally, for anyone interested in Solti's pioneering studio recording of The Ring, John Culshaw's book 'Ring Resounding' is not only self-recommending, but perhaps one of the most engrossing books I've ever read.