According to the RAW (in OSE at least), an individual only has to cast Read Magic on a spell scroll or magical inscription once to be able to understand it. The ruling for spell books is weirdly vague, but I would rule that you need to cast Read Magic every time you want to understand it; to me, that's a big part of why copying from a spell book takes so long.
Also, for the future, note that copying a spell has a chance of failure based on the copier's Int score, with failure meaning that "the character can never learn the spell." That seems a little harsh to me, so I want to very creatively interpret that rule to say that failure means you can never learn the spell from that source. If you really wanted to learn, say, Fire Ball, but you failed to copy it from a scroll, you would have to find another one. You wouldn't be permanently unable to learn Fire Ball.