Answer: First, you need to dissect the excerpt.
1. I can hardly describe to you the effect of these books.
This means that the books have left the speaker/reader speechless.
2. They produced in me an infinity of new images and feelings that sometimes raised me to ecstasy
This means that the images put bright ideas, or other images in their head, and feelings that were not thought of until those book were read. To be raised to ecstasy would mean to be flooded with a good feeling.
3. but more frequently sunk me into the lowest dejection.
This means that more than often these books, often made the reader sad, the books broke the reader's heart as they were reading the book. They made the reader fall deeper and deeper into despair reading the book.
Step-by-step explanation: