I read a couple :
- Ken Schramm's The compleat meadmaker
- Roger Morse's Making mead
- Harry Riches Mead Making, Exhibiting and Judging
- Pam Spence's Mad about mead
If you look in the posts on the forum, You'll find that Ken Schram's book is not only one of the most up to date meadmaking books available, but also regarded as one of the most extensive in its coverage.
I have to second that fully, if I must recommend a single book it'd be Schramm's.
They all carry interesting pieces of historical, technical, and something philosophical wisdom, but that one definitely stands out.
Please note however that there is a lot of information on GotMead? that is more current that what can be found in some of these books. Newer techniques such as nutrients addition schedule, which are easy to implement and very efficient are better learned on this site, until a revision or newer books get printed.
(NB This is just a personal advice, I can be wrong and don't hold 'the truth'

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