I am not even sure whether this is worth the effort. But if you want to check it out, there are plenty of titles available
1. Open ibooks in your iOS device (Not in mac/itunes store) and select 'store'
2. In the featured tab, scroll down to 'Popular Collections' and select FIRST IN SERIES and all the books are free in that collection
I don't have a link for this because if I open this in iTunes or iBook for Mac, it is showing me different results. So in the URL, I am putting the link to the ibooks application
UPDATE: Thanks to Slowy for the link
https://itunes.apple.com/WebObjects/MZStore.woa/wa/viewMulti…
Worked for me on my iPhone thanks!