A book, in which you can read interesting things, is more thrilling than TV.
It all depends on what you mean to say.
If you want to imply that all books are interesting to read, then the relative clause has to be non-restrictive or non-defining (set off by commas):
- A book, in which you can read interesting things, is more thrilling than TV.
If you want to imply that not all books are interesting and that only those that are are more exciting than TV, then you need a restrictive or defining relative clause (without commas):
- A book in which you can read interesting things is more thrilling than TV.
Depends on what you want to convey. The comma used by you can be understood as a comma with an appositive.
Appositive is a word or word group that defines or further identifies the noun or noun phrase preceding it.
Rule: When an appositive is essential to the meaning of the noun it belongs to, don’t use commas. When the noun preceding the appositive provides sufficient identification on its own, use commas around the appositive.
Example: Jorge Torres, our senator, was born in California. Explanation: Our senator is an appositive of the proper noun Jorge Torres. Our senator is surrounded by commas because Jorge Torres is a precise identifier.
Example: CEO Julie Minsky will be our featured speaker. Explanation: Julie Minsky is necessary to help identify CEO, so no commas are used.
In your example, "In which you can read interesting things" is an appositive.
Now, you see the usage of the comma can be deemed correct or incorrect based on whether the appositive is essential or a mere ancillary descriptor. You be the judge. What do you want to say? If you are implying that all books have interesting things to read then Book is a precise enough identifier and the appositive is ancillary. So, use the comma. On the contrary if you feel that only the books that have interesting things to read are more thrilling than TV then, "In which...things" is a necessary descriptor as to what Kind of books you are talking about so you should skip the skip the comma.
A comma usually signifies a pause. Read the sentence out loud, pausing after each comma. Now read it out loud without the commas. Your ears should tell you how the meaning is affected.
I think you mean to say, "An interesting book is more thrilling than TV." If so no commas. But you sentence will be awkward. If you intend to say, "A book is more thrilling than TV because you can read interesting things in a book," then you need the commas. Your sentence will nevertheless be awkward and the proposition will be dubious.
I respectfully urge you to minimize commas by rephrasing your sentences. Your writing will be clearer and flow better.