All the free ebook sources have been mentioned. I can only add a suggestion. I will qualify it with I know there is no extra money right now. Despite that fact I do have a suggestion which might make it a lot easier for you. Plus it will help stretch the lifespan of your laptops.
I would suggestion, and mind I am not forgetting you are a broke-arse person these days. Trust me, I have been there and will prolly be there again sometime. Breakfast, lunch & dinner made of some ramen noodles, an egg, a bit of cheese and if you are lucky the grocery store has some old snake legs being sold under the guise of chicken legs.

But if you can garner around $50 over the next whatever time-frame start looking for an older PDA type device with a transreflective display. I am a fan of the Clie devices from Sony as I have an PEG-NX80v that I bought a while back used on eBay for about $50 because the battery did not work. $8 for a new battery and I have used it daily for almost 2 years now.
Admittedly I got a steal on mine but there are other brands and devices which all can be had for under $50. I know the trick would be finding someone to ship to you with the cheapest possible mail class. If you can get someone to ship it in a small box and it weighs under, I believe the limit is 4lbs., First Class is not going to be very expensive. A 2lb 1st class parcel from the US will be around $17 for just the postage and hopefully there would be no duty and or VAT of a device older than 4 years. Or maybe you might find something on the eBay UK site.
A PDA with a transreflective display lets you read outside if you like. Just know not all transreflective displays are created equal and may not all be as readable outdoors. I forget but one of the Palm devices, maybe the III (?) can be found dirt cheap at under $20US and would solve the issue for you for a while. But there are just oodles of great undervalued PDA devices to be had for a song with years of life left in them. Plus they have all the PDA functions on top of being very decent reading devices. Sure they will only last for a few hours of reading but that is far better than no reading or even lugging around your 17" laptop. I know because I have one, even though I do take it everywhere.
This thread and the forum in which it resides might give rise to some practical and affordable options. And remember eink is the the cure for life.
https://www.mobileread.com/forums/showthread.php?t=21582
And if you do consider adding a Clie search out one with the jog wheel on the side. It makes one handed reading a pleasure in portrait orientation. There are several which have the wheel there and several with this tool located elsewhere. I read a thread somewhere here with a discussion on that very topic.
If reading really is you only sort of escape and recreation from daily stresses, finding a way to budget for one should be a priority. To quote a favorite character of mine, Lazarus Long, "budget the luxuries first". I always took that to mean, life is far too short and if one is merely surviving, it is not going to be much point any of it.