Noticed that for SY0-501 books, Darril Gibson's book (https://www.amazon.com/CompTIA-Security-Get-Certified-Ahead/dp/1939136059/) is listed as "the favourite book of this sub" but for SY0-601, his book (https://www.amazon.com/CompTIA-Security-Get-Certified-Ahead-ebook-dp-B09237T9ZB/dp/B09237T9ZB/) is not even listed. Is the above list outdated or is his SY0-601 book much worse than his SY0-501 book?