I have "Type 1 Diabetes" by Dr Ragnar Hanas, it was recommended me by someone on here - it's pretty hefty & covers everything you need to know. Our diabetic nurse says she uses it as her bible, so it must be a good 'un! It's a big book, rather than read it cover to cover I dip into it for bite sized chunks every now and again.
Type 1 diabetes in children. adolescents and young adults by Ragnar Hanas is an excellent book and was very helpful to me in learning more about diabetes and how to manage it. You can buy it at Amazon.co.uk for around £15 for the old version (blue) and £17.50 for the new version (orange), well worth the money.