Required reading: 

Buchan, D.; Keay, M. (2015): Europe's Long Energy Journey: Towards an Energy Union? 32-62.

Gustafson, T.(2020): The Bridge: Natural Gas in a Redivided Europe, 184 - 215. 

Supplementary reading:

OIES (2021): Oxford Energy Podcast – Why are gas prices so high? Oxford Institute for Energy Studies. https://www.oxfordenergy.org/publications/oxford-energy-podcast-why-aregas-prices-so-high/