Curiously, all three of these are correct: 1) The book was so good that I couldn't put it down. 2) The book is so good that I couldn't put it down. 3) The book is so good that I can't put it down. But this is impossible: 4) The book was so good that I can't put it down. 3) is only used when you still reading the book. For example, I am in bed and I wake up at 2 a.m. and notice that my wife's bed lamp is on and she is reading. I say "Wow, honey, you're up late." She replies "The book is so good that I can't put it down." 1) and 2) are used in the more likely situation in which you've finished the book, and are writing a book review or recommending it to a friend. 1) seems more logical. The reason why 2) is acceptable is that being good is an intrinsic characteristic of the book. It is either a good book or it is not. If it was good when you read it, it is still good. Therefore it is OK to couple "is" a good book with "couldn't" stop reading it. The fact that I "couldn't" stop reading it is the proof that it "is" a good book. Another example of this pattern would be "the hill is so high that I needed to rest twice when I was climbing up it." So, when you are praising a book that you want someone to read, it is natural to say "this book is so good" because putting it in the present makes it feel stronger and more immediate. 2) isn't better; people who are very precise in their use of language might feel that it is very slightly wrong.

Very simply, it just depends if you have finished the book or not. Can't if you are still reading and couldn't if you have finsihed

In the sentences with is - can't you are referring to a book that you're currently reading and have not finished. In the case of was - couldn't you are talking about a book you read in the past and that you've finished.