It's time limited to read AFAIK so you have until March 12 to read them.
If you want to keep them... Better startup your screencaping software since bookwalker books are impossible to grab into pdfs. (Or better yet, buy them when the next 40% points-back appears. Too bad they don't do 40%off+40%points anymore.)
Yuyake Koyake and Gundog are both pretty good though (I actually have all 3 versions of Gundog for some reason? And there's a Girls' Frontline doujin conversion out there as well.)