A book that I desire to delve into more deeply is Conybeare and Stock’s Grammar of Septuagint Greek. The grammar itself is short, and still leaves to be desired a fuller grammar of Septuagint Greek. But the selected readings have Septuagint text with reading helps at the bottom of each page, and make for a nice bridge into reading Old Testament Greek. The readings section looks like this (click to enlarge):
You can access the whole book for free (and legally) here. The format there is a little unwieldy, so you might also consider checking it out in print (affiliate link).
The grammar is pretty dense, and feels sparse in places, but I haven’t had a chance to examine the book thoroughly yet. The selected readings have helped me in improving my Greek, though. It’s recommended reading on this Septuagint Sunday.
Words on the Word 
$10 in Logos, too: https://www.logos.com/product/28122/grammar-of-septuagint-greek
I didn’t realize the Logos edition had the selected readings portion, too–cool! I would need to double check this, but I think that its edition in Accordance and BibleWorks only has the short grammar.