Should present tense be always used when completed works / published books etc. make reference to their contents?

It depends what you want the sentence to mean. Tenses are there to provide information.

Example

The book contained valuable information. (This means the book did contain the information in the past but now perhaps the writing has faded or the book has been destroyed)

The book contains valuable information. (The book is still in existence and still contains the information)

The book is very helpful to teenagers. Teenagers read this book and find it helpful.

The book was very helpful to teenagers. Nowadays the book is no longer helpful because it was written in Victorian times.

The book will be very helpful to teenagers. Teenagers who read this book in the future will find it helpful.