The end of Agatha's book is the Darkhold. WandaVision used the book to explain how Wanda Maximoff is the Scarlet Witch, and she ends the series by studying its teachings.
As exciting as it was for many to see the Darkhold in WandaVision, those who watched previous Marvel TV offerings had questions. The Darkhold originally appeared in Agents of SHIELD season 4 and played a prominent role in the Ghost Rider plot. He was the last one to have it until the Darkhold's brief return in Runaways season 3. The use of the Darkhold in WandaVision caught Agents of SHIELD fans off guard because it looks completely different now. This raised even more debate about whether or not Agents of SHIELD fit in the MCU and could be another example of Marvel Studios retconning what happened. However, WandaVision director Matt Shakman weighed in and suggested this may be the same Darkhold as the one in Agents of SHIELD. While that could be viewed as a way for Shakman to avoid saying definitively that the ABC superhero series isn't canon, Marvel Studios could make it work.
Even though the Darkhold in WandaVision and Agents of SHIELD (and Runaways) had a different appearance, Marvel has not explored the origins of the book. The comics explain that the powerful Elder God named Chthon created the Darkhold thousands of years ago to obtain more power on Earth as humans used his dark magic spells. This could be kept for the MCU and be a way for WandaVision and Agents of SHIELD to have their own versions of the Darkhold. If the Darkhold has been part of the MCU for hundreds or thousands of years, it would make sense for there to be different copies, especially with its wealth of power. The comics even have pages of the Darkhold copied inside the Book of Cagliostro, so the MCU having two copies of the book itself can work.
If the MCU eventually reveals WandaVision's Darkhold is different from Agents of SHIELD's version, how and why could there be two copies? We don't know when Agatha came into possession of the Darkhold, but her origins date back to, at least, 1693. It's possible she has had the Darkhold for hundreds of years and was studying it to increase her power. If Chthon created the MCU's version of the Darkhold to further his power, though, he might not like that his book isn't being used to its full potential. Chthon could've created new versions of the Darkhold, such as around the time Wanda was born so that there was a better chance of him gaining an influence over the Scarlet Witch. It's also possible that a disciple of Chthon, the Masters of the Mystic Arts, or someone else who had prior knowledge of the Darkhold made a second version if the original couldn't be found.
The MCU could get around the WandaVision and Agents of SHIELD retcon in other ways too. Marvel could explain that a magical book can change its appearance. However, this would still mean the shows' timeline of events has to match regarding the Darkhold. There isn't anything in WandaVision that directly contradicts Agents of SHIELD's use of the book. Still, this explanation means at some point Agatha Harkness had to take it from Ghost Rider. It would frankly be infinitely more surprising if Marvel Studios used this rationale. Not only is it more complicated, but the studio has so far done everything it can to avoid tying to Agents of SHIELD. Should the Darkhold appear in Doctor Strange in the Multiverse of Madness as expected, perhaps this will be when the MCU confirms how or if there is a connection.