Question about the LNs and Anime

I've just finished watching Owarimonogatari (I watched the series in airing order) and was thinking of reading the light novels before watching Koyomimonogatari (yes, it's weird but I'm sticking with it).

Any idea on which LN I should start reading so as to not "repeat" what the anime series has shown (don't really want to waste my time "re-reading" what I already know)?

And on the note on LNs, any eBook sites to recommend for me to buy them from? Amazon Kindle doesn't have this series and I don't feel comfortable going to those shady "free ebooks" website.

Thanks in advance!