On the subject of seed saving my first go to book is a translation from German to English of "The Manual of Seed Saving" which is a book compiled by Arche Noah and Pro Specie Rara which are Austrian and Swiss non-profits dedicated to plant species preservation. While originally written in German, I find the English translation to be excellent. The book focuses on edible garden plants and to my taste strikes a good balance between "how to" and "why". Being a bit Eurocentric there are a few plants covered that you may not have heard of before but I take this to be a good thing.