The difference is extremely subtle here. Use of the present continuous will emphasize these two things 1 it is an activity (reading a book) and 2 it is while that activity is taking place that you find yourself in a state of bliss.

Hello Jacky, Your question should contain the word 'difference' not 'diffidence.' These words have different meanings. I find bliss when I read an interesting book .... (present tense) I find bliss when I am reading an interesting book..... (present continuous) Some people have suggested it could be 'read' (pronounced 'red') in the past tense but that is not possible grammatically. 'Find' would have to change to 'found.' I found bliss when I read a book.... The difference between the present simple and present continuous here is insignificant. Really there is no change in meaning. Use of the present continuous perhaps emphasizes the action of reading, its continuous nature, the duration of the event, and that it is during the reading that you find bliss. But the difference is small, and some people would say there is no difference in this situation. Let's take a different situation, and maybe we can see the difference better. I get tired when I walk up hills. I get tired when I am walking up hills. In the first it looks as if my tiredness could be during or after the climb. In the second (present cont) sentence it is during the climb that I feel tired. I probably am tired after too! But it emphasizes the action and the hills and the 'getting tired' during the walk. But it is just emphasis. A speaker might use either form, and there may be no significant difference intended.

"When I am reading a book" is a present tense, basically your present situation. "When I read a book" is a past tense, when you read it and done with it in a present situation.

The word "read" can be past or present tense, depending on its pronunciation. To me, it looks like, "When I am reading a book" You could be talking about the past or future, like telling a story. "When I read a book" Pronunciation: Reed. Same thing as above. "When I read a book" Pronunciation: Red. It happened in the past.