Since you don't get into combat early very often, you neither need that hp to Proxy, nor do you need the damage fighting any enemies from Afterhock, so refillable, time warp tonic and dark seal (+Corrupting and Ult later) are providing more than enough HP Regeneration and sustain through Mana Regeneration to safely continue with your Proxy after trades (which you outheal afterwards).
Spellbook is insane for later situations where you are able to quickly change to the needed summoner spell and get easy lockdowns or solo kills on laners, while keeping the much needed TP.