With following doctrine we have to remember that 500 BC India was a radically different social and economic landscape that today and many ways of being in the culture of the Buddha's time just do not overlap neatly onto ours. That said, I have read many books, usually by Theravedins that state explicitly at the front of the book that its not to be sold for money and should be returned to monastery if found. However, it took resources to print and bind those books and nothing is "FREE" in this current version of the world, being that its a capitalist oligarchy. We're all wage slaves and beholden to the Taxman, be a pal and donate something in return for the Dharma. In this climate, the Dharma can't proliferate as well without cash to do so.
There is however, a large amount of texts online @ sites like https://read.84000.co/ and http://buddhanet.net/e-learning/index.htm Also, if you really dig in like a scholar, you'll learn the names of certain important texts like the Visudhimagga and the Bodhicharyavatara, the Mulamadhyamakakakarika which usually have a pdf somewhere on net, Try Google. Dharmawiki and Rigpawiki are amazing resources for looking into terms and concepts along the way.
My advice is to start digging into the key texts in the Tripitaka, then move forward, trace how Buddhism spread to different parts of the world, taking on specific characters, and you'll find the relevant teachers and their works as they happened. Lots of armchair Buddhists bicker and argue about the doctrinal validity of such and such a text, the veracity of Mahayana texts, the legitimacy of one sect over another etc. Don't listen to any of those holier-than-thou boobs. Read it all.
Also, while there are some gems hidden in the rough, I'd suggest to not expect much or put much stock into contemporary Buddhist books. Most have been dumbed down, stripped of anything too esoteric or challenging and are more in line with Pop psychology than anything else. Lots of people will argue this point by saying that you can still get wisdom out of modern Buddhist thought, which may be true, but its almost exactly like suggesting you can become good at Math only by studying basic arithmetic and never venturing beyond the grade school level. I think this tendency comes from our collective inability to pay attention and our need for instant gratification. I call it "Bumper Sticker Buddhism" because most of what they say are little simplistic platitudes that can fit nicely within the confines of a sticker.
Good luck! I'm not much of a practicioner, but I am well read, so if you have any questions or want more recommendations, shoot me a message.