Any gardening book - so she can share Derek's interest in gardening and develop a better interest in it for herself.
A good wildlife book on creatures native to the UK - again this is to develop Eileen's knowledge.
The Lord of the Rings - partly because Eileen feels she should give this a try but mainly to see what an epic quest that doesn't involve her looks like.
Joy In the Morning - Eileen is also keen to develop a sense of humour (it will make her seem more human) so has rightly assumed reading the works of P.G. Wodehouse will help here.