It is helpful to start with an understanding that some verbs denote an action that has limited duration and a clear completion point. A term for such verbs is punctual. For example:
I have bought two books on Amazon since 9.15.
If a punctual verb is used in a present tense expression with since, then the action cannot still be continuing. In other words, the actions were completed at a time between 9.15 and the present.
Other verbs denote an action that takes place over a period of time. These are called durative verbs. For example:
We have taught at the school since 1965.
With such verbs it is open to interpretation whether the action is continuing or not. We could still be teaching at the school or we could have taught at the school for a limited period in the years since 1965.
The first interpretation (still teaching at the school) seems much more likely. But to rule out any possible ambiguity, the continuous aspect could be used:
We have been teaching at the school since 1965.
The same applies to They have been at the hotel since last Tuesday. It is considerably more likely that it means they are still there. But in this case there is no possible continuous alternative:
*They have been being at the hotel since last Tuesday.
To avoid any possibility of ambiguity you could replace to be with to stay:
They have been staying at the hotel since last Tuesday.
Sentence 1 (I have studied two books since 9.15), is similar to the teaching example in that both interpretations are possible. But in this case the two interpretations seem of roughly equal likelihood†. Again, if you want to make it clear that the studying has continued from 9.15 up to the present, then it is best to use the continuous form:
I have been studying two books since 9.15.
† The equal likelihood may be because to study can also be regarded as an action with an completion point, For example: I have studied Keynes, so I know something about macroeconomics.