eBooks or not to e

I find myself on holidays for a week with plans to read, relax, walk on the beach and have tea with friends... I've set up my new PDA with mobibook so I can read ebooks. So far it's been great for me - the benefits have included carrying less books/diary etc in my handbag, reducing my luggage, being able to listen to music while I read.... but now I'm nearing the end of my very enjoyable read, Kathy Reichs - Deadly Decisions, I'm not quite sure how I can give my 'used' book to my mum? I thought going e would be environmentally friendly, but now I'm challenged with my sense of sharing with my community. Being able to reuse books by sharing them is also a strong sustainability characteristic and its also about contributing to comunity. If any one has any ideas on how to share eBooks ethically, let me know. The jury is still out 'e or not to e'??