Mere Christianity by C.S. Lewis

[Goodbook]

Thats good you are praying for your sister, hope you will find the right book for her. I haven't heard of On Guard.
One I heard that was popular for skeptics and apologists was 'the case for christ' and quite readable, but one I found that helped me on my journey actually was Nancy Leigh De Moss 'Lies young women believe'.

 

[Quid est Veritas?]

Mere Christianity started me on the path from atheist to Christ my Lord and Saviour. Its not for everyone, but it is a magnificent book, I highly recommend it.

 

[miss-a]

It's an excellent book, especially for someone who has an active mind that needs a lot of answers. Lewis was a logician, so he helps people who find spiritual topics illogical see how logical it really is to have a relationship with God through Jesus.