Must is not an ordinary English verb. It's a Modal Auxiliary Verb, which are very irregular.
One of the strange facts about modal auxiliaries is that they always have at least two kinds of meaning. One use of modals (like must, should, or may) is called the Deontic sense, and it always deals with social permissions and obligations. All of the following are deontic, and have to do with what you are obliged or allowed to do, in the present or future:
- You must remove your shoes before entering.
- You should study harder.
- You may leave after finishing the exam.
The deontic sense of must is the meaning that the English grammar book was talking about; obviously you can only have an obligation to do something in the future, so deontic must can't be used in the past.
However, the Epistemic sense, the other meaning of modal auxiliaries, works just as well in the past as the present, since it doesn't refer to social necessity, but logical necessity.
Present epistemic (indicating a logical conclusion about the present time):
- This must be the place he told us about.
- He should be home by now.
- That may be true, but there's no evidence.
Past epistemic (indicating a logical conclusion about a past time):
- That must have been the place he turned East.
- He should have been home by then.
- That may have been true then, but it isn't now.