I think second to picking equipment, spell selection is the second thing that slows character creation down. So I looked through the level 1 spells and created themed spellbooks. I tried not to repeat any of the spells so each book is unique. The addition of the level 2 spell is to incentivize getting there.
Interestingly, while Readings on Astral Combat contain the most combat-oriented magic and therefore the most powerful, I think the other three books actually solve problems that might stimy level 1 BX PCs or lead to the greatest reward from an early dungeon.
Its might also be interesting to write a dungeon using obstacles implied by level 1 spells.