So, on the countless side materials, spin-offs and complementary content Steins;Gate has, you must be curious which ones are worth reading. While I'm not a real expert on that, I do love the series to its fullest and therefore recommend you this spin-off manga.
A lot of the complaints from Steins;Gate's last episodes were the monumental slap Mayuri gives to Okabe, and how uncharacteristic that action is for a character such as her. It seems that the creators acknowledge this mistake, and produced a Drama CD in 2010 that acts as a backstory behind Mayuri's sudden change of character—and a tale worth reading for any fan
of the Steins;Gate franchise.
Of course, this manga involves *major* spoilers for Steins;Gate, so you shouldn't read this unless you have finished watching the anime or playing the VN. In the Beta world line where Okabe fails to save Kurisu, Mayuri prevents Okabe from traveling back in time once again and causes this significant divergence where the future is looking grim with World War III. The series takes place a year after that eventful day, and takes a look at Mayuri's life with a PTSD-ridden Okabe.
This being a short manga, I really can't say much more than recommend this for anyone who wants more Steins;Gate content and can't wait for Steins;Gate 0. As a side story, it fleshes out a significant amount for Mayuri's character and relives the emotional moments that Steins;Gate is known for—taken from a different perspective.