Depends what level you are already at? Most regard A Guide to Bees & Honey by Ted Hooper is No1. I think it is up there but not as accessible as some texts, when reading him first time round I found that he wrote with an assumption of prior knowledge in his readership that I simply didn't have, so his wisdom was lost on me.
My recommendation >>
A Practical Manual of Beekeeping by David Cramp. It has given me the confidence to undertake most inspections and manipulations of my colonies during the first year and just as importantly, when I have returned to other texts - Like Hooper I have then understood the text and taken the benefit. That may be a combination of practical experience as well as prior reading but I recommend this book for a structured, approachable, informative and at times amusing style. I have read fairly widely now and for me this book rates No1 for new to intermediate readers as a good all round reference and introduction.