The Issue

Because reading is good removes the skill bonus from skill books in favor of a % skill gain boost, Xarxes loses his Follower buff. 

The Solution

Now, instead of nothing, Xarxes boosts the experience gain of skills in the same way that Reading is Good does. This applies retroactively - meaning that it also counts books you've read before becoming a follower of Xarxes. In addition, it also stacks with the base boost that Reading is Good offers.

The caveat

The boost stacks multiplicatively with reading is good instead of additively. This is done to avoid extraneous edits to Reading is Good and its scripts, and to reduce the complexity of the scripting.

Assuming default Reading is Good values, this means that if you have read, say, 5 Conjuration books, the boost from Reading is Good will be 20% and the boost from this patch will be an additional 20%. The total buff will be 44% instead of 40%.

The Technical Stuff

This patch can be installed during a running save.
This patch is not safe to uninstall.
This patch is flagged as an ESL and does not count toward your plugin limit.
This patch applies its effects retroactively, so long as Reading is Good was installed when you read the skill book. For the love of god use a mod manager.
You need both Wintersun and Reading is Good.

Acknowledgments

Thank you to Parapets, because without the Papyrus API this would not have been possible to make.
Thank you to the xEdit team, which made it significantly faster to create this mod.
Thank you to everyone who has helped me improve my scripting over the time I've been modding.
Thank you to EnaiSiaion for Wintersun.