優しい means kind, gentle, tender, soft, mild, etc. it's used to extress personality, taste, flavour, texture, etc. 易しい means easy. We use 簡単な, 楽な, ゆったりとした depending on the things which we are talking about. *I prefer write やさしい with hiragana, however, it's only my liking.

ありがとう！thank you very much!

この本は優しい is incorrect. 優しい(yasasii) is kind. Easy is 易しい(yasasii). Same sound. So この本は易しい is right.