It is considered a "top" resource because, unlike the official AUTOSAR standards (which are thousands of pages of dry specifications), this book condenses the information into a practical, digestible format for engineers.
Without the foundations laid out in this PDF, developers tend to treat AUTOSAR as a "black box" configuration tool. With the Compendium, you understand why the RTE generates buffers or why the COM stack needs a PDUs (Protocol Data Units).