5UIN - Employee On-boarding literature recommendations

Hi All, 

I am currently studying my CIPD level 5 and am in the process of completing 5UIN (using information in business). I have decided for my assignment, to do a critical review of our employee on-boarding process and it's impact on productivity/retention of new starters. 

Can anyone recommend any literature that would be beneficial to read to aid my lit review? 

Many thanks in advance!