I have never heard that "t" is silent in the word "dentist". I would say it is not a norm of the language. For the same reason "t" remains in the standard pronunciation of the word "context". The rest is dialect.

I don't think this is something you can learn from a book. It takes exposure to the speech patterns of American speakers. In American pronunciation the T after N can be silent in relaxed speech or as the result of intonation -- if the T precedes an unstressed lax (short) vowel. interview...can sound like... innerview winter ....can sould like....winner dentist....can sound like ...dennist semantic...can sould like....semannic ---- In many words after N the T is followed by a lax vowel that receives a secondary stress. In these cases the T is pronounced. context.....the e has a secondary stress so it sounds like con text content....the e has a secondary stress so it sounds like con tent dentist....if you stress the i sounds like den...tist semantic....can sound like se man tic cantankerous.....sounds like can tan k'r's ----- However, in speech with careful enunciation, in ALL situations it is possible to pronounce the T for the sake of clarity. It was a winner of a winTer. In this case you would naturally pronounce the T for the sake of clarity.

Uhm.....I couldn't have found your answer yet.