According to Amazon it's being released in September.
https://books.disney.com/content/uploads/2019/06/Loki_Mackenzi-Lee_Excerpt.pdf
Loki: Where Mischief Lies https://www.amazon.de/dp/136802226X/ref=cm_sw_r_apan_i_ZMS5YD3Q1R7SYCXN7X9M