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«Atomic Habits», by James Clear, is not a finance book, but its central idea is a first-rate financial tool: you don't rise to the level of your goals, you fall to the level of your systems.
Wanting to save is a goal. An automatic transfer of €100 the day you get paid is a system. The goal depends on your willpower, which runs out; the system works even in the months when you fail.
Clear also proposes changing your identity before the outcome: not «I want to save», but «I'm someone who doesn't spend what they don't have». And playing with friction: if buying is one click, keep the card two drawers away. One percent better each day seems like nothing; in money, it's the difference between arriving just barely or arriving with room to spare.