2. Rhaenyra was not best friends with Alicent
In the book, the 10-year age difference between Rhaenyra and Alicent means that they are never best friends. They are only loosely acquainted before Alicent marries Viserys, after which they’re on friendly terms; in the book, Rhaenyra even pours wine for her new stepmother at her wedding feast, and Alicent kisses her and calls her “daughter.”
Things change when Alicent bears Viserys a son, at which point their relationship deteriorates. On the show, they already have a close friendship when we meet them. It slowly degrades over the years into a tragic and heartbreaking rivalry.
3. Rhaenyra’s affair with Criston Cole is a gray area in the book
In Fire & Blood, the idea that Rhaenyra and Criston Cole had an illicit affair is a rumor started by the dwarf Mushroom, a fool in the Targaryen court who does not appear on the show. There are different versions of the gossip; some say Rhaenyra rejected Criston’s advances as happens on the show, while others say Criston rejected Rhaenrya.
Again we have an age difference. In the book, Criston is 16 years older than the princess when they supposedly get together, after becoming her sworn protector when she was 7-8. The book never confirms whether they slept together, but it mentions Criston asking Rhaenyra to elope, which she declines. The show blends together different versions of the rumor. But it does leave out the part from the book where Rhaenyra questions Criston’s ability to keep marriage vows after he readily broke his Kingsguard vows to sleep with her.
Rhaenyra refusing Criston’s proposal is an example of her stepping up into the role of heir apparent to the Iron Throne; in that role, she’s expected to marry someone from a powerful family who can advance the interests of House Targaryen, not a member of the Kingsguard. Casting doubt on his loyalty as a potential husband would’ve been a great addition to the scene to show Rhaenyra’s growing shrewdness.
4. Rhaenyra was good friends with Laena Velaryon
In House of the Dragon, Rhaenyra Targaryen is not close with any female character except Alicent. However, in the book, she is “fond and more than fond” of Laena Velaryon, Daemon Targaryen’s second wife. They frequently saw each other when Laena was married to Daemon and even flew together on their dragons.
In Fire & Blood, Rhaenyra betroths her sons Jacaerys and Lucerys to Laena’s daughters, Baela and Rhaena when all of them were kids between the ages of 2 and 4. The decision is not a desperate attempt to appease Rhaenys Targaryen like it is in the show.
In the book, Rhaenyra is also close with some other ladies-in-waiting at court, including Harwin Strong’s sisters and Elinda Massey.