Mosul - Receipts from taxi rides, ledgers listing internet usage for the privileged few and random logbooks documenting an ever tighter economy are just some of the documents that Islamic State militants left behind when they fled eastern Mosul in the face of advancing Iraqi forces.The discarded papers and bundles of receipts, found on a recent visit to a home used as a base for the militants in the city, offer an unusual glimpse into the Islamic State group's daily life and economy.In the months leading up to the Mosul offensive, ISIS fighters were increasingly pushed underground by punishing US-led coalition airstrikes.The bookkeeping reveals how ISIS bases had become increasingly like bunkers, but also how easily the fighters were able to move within their so-called caliphate just a year ago, when it spanned across western Iraq and a third of Syria.Most of the receipts were from early 2016, when ISIS had only just lost control of the city of Ramadi in western Anbar province, but still controlled about a quarter of Iraq's territory. Slips of paper document taxi rides back and forth to ISIS-held towns across the Iraq-Syria border.According to the receipts, Hit was a frequent destination - a small crossroads town along the Euphrates River that was an important logistics and supply hub for ISIS. The six-hour drive cost only 29 000 Iraqi Dinars or about $22. The drives were likely runs to pick up supplies or hold operational meetings.Another slip of paper on ISIS stationary bills a Mosul-based passenger, likely an ISIS fighter, for fuel purchased in the Syrian city of Raqqa - the de facto capital of the ISIS group.Stacks of papers also testify that the group kept close tabs of utilities such as electricity and internet usage. Monthly cards bearing users' internet names and passwords were filed with the base's expenses.While internet and mobile phones were strictly outlawed under ISIS in Mosul to prevent civilians from becoming government informants, the internet receipts suggest ISIS used centralised internet connections across the city.ISIS-held territory in and around Mosul and in Anbar has significantly shrunk over the past months - the roads fighters once easily travelled by taxi in early 2016 are now dotted with government checkpoints and airstrike craters.The western half of Mosul, which is still under ISIS control, is almost entirely cut off from territory the militants hold in Syria. In Mosul's east, the abandoned ISIS bases sit ransacked by security forces, intelligence officers and curious neighbours.