"I was reading a book at 7 last night" looks fine to me. However, if you want to use the word "read", I would remove the"was" from the sentence. It would look like this, "I read a book at 7 last night.".

Yes, both I was reading a book at 7 last night and I read a book at 7 last night are OK, but the first one is more natural.

Yes, Sarah's explanation makes sense, and is why the first is more natural. It doesn't make the second wrong, it's just a little odd.

Thank you so much, Sarah!

Helen，我刚刚回答过你另外那个问题，和这个问题很相似，不过回到这里来给你解释发现你已经选好答案了～ 这个问题上我和下面的回答者想得不一致，我认为只有"I was reading a book at 7 last night"才是正确的。这是因为看书这个事情不是一瞬间就能完整的，它需要一段时间才能做好。别的一瞬间就可以做好的事情可以用这样的说法，比如: I came home at 7 last night (你到家那一刻只是一瞬间), I left at 6 this morning, I woke up at 7 this morning 等等。在一些比较特许的情况下，需要一段时间的动作也可以这样说，比较常见的一个例子是：We ate dinner at 6 last night，虽然吃饭这个动作需要一段时间才能完整的，但我们可以只说开始吃饭的那一瞬间。不过大多数的时候需要一段时间来做的事情需要给一个时间范围才能用简单过去时态，不给时间范围的话用 was ...ing 还是比较好。 不知道这样解释是不是清楚，有问题请随时提问～～