Anko Mitarashi could be a good candidate. She's strong -willed and has her own set of skills. Her somewhat wild personality could contrast well with Kakashi's more reserved nature. They could have met during a mission and over time, developed feelings for each other.
Yes, there are some. You can check out fanfiction websites like FanFiction.net or Archive of Our Own. Just search for 'sakura wife kakashi' on those platforms and you'll likely find a variety of stories. Some might focus on their relationship development, while others could be more adventure - based with their relationship as a side - plot.
Often, Naruto is paired with Kakashi in such fanfictions. Their bond from the original series can be explored in new ways in the context of Kakashi's reincarnation. For example, Naruto might be the one to discover Kakashi's reincarnated form and help him adjust.
Often, Gai is paired with Kakashi in Hurt Kakashi fanfiction. Gai, with his unwavering belief in friendship and his eternal rivalry with Kakashi, can be a great character to interact with a hurt Kakashi. He might try to pull Kakashi out of his slump through their friendly competition. Also, Rin is sometimes included. Since she has a significant past with Kakashi, her presence in the fanfiction can either be a source of comfort or more pain, depending on how the story is written.
Naruto would probably be one of the first to try and rescue Kakashi. He has a strong bond with Kakashi and would not stand by while his teacher is in danger.
Naruto is often a common character. Since Naruto has a strong bond with Kakashi, he might be involved in Kakashi's time - traveling adventures. Sasuke could also be there. Their relationship with Kakashi in the original story makes it likely for them to show up. And of course, Guy would be a great addition. His friendly rivalry with Kakashi would add some humor and action to the time - travel scenarios.
Sasuke could be a popular character. He might be involved in the story in a way that either helps or hinders the situation where Naruto hurts Kakashi. For example, if Sasuke tries to stop Naruto from doing something stupid that would hurt Kakashi.
I think Rin would be an interesting character to pair with Kakashi in time travel fanfictions. Since she was an important part of Kakashi's past, their relationship could be explored further. Maybe Kakashi travels back to save her from her tragic fate, and they end up having a different kind of relationship as a result.
Archive of Our Own (AO3) is also a popular option. It has a tagging system which makes it easy to search for specific types of fanfictions like 'sakura wife kakashi'. There are often detailed descriptions and reviews to help you decide which ones to read.
There is no way to say for sure who the author of the 'kakashi book story' is. It could be a new author trying to make a name for themselves or an established author writing under a pseudonym. Maybe it's a story from a small press or self - published, in which case it would be even more difficult to find out who the author is without further research.