Well, first of all, any primary source is free if you can take the book out from the library! For actual scholarly research, this is probably preferable to the free online materials, since these tend to be older translations that are now in the public domain. Thus, for example, while this online text of William of Rubruck has been checked against the more recent translation by Peter Jackson, Jackson's would still be better for academic work if you can source it from a library.

That said, even if the translations can be a bit dated, there's a great amount of material out there. This collection (which includes the online Rubruck linked above) is a great place to start; Henry Yule's four-volume Cathay and the Way Thither is another, though I think it's considerably denser. (Both of these also contain materials about Buddhism and Hellenism on the Silk Roads—for the former, see the monastic accounts in the University of Washington site, and for the latter, vol. 1 of Yule.)

Obviously the topic is super broad and my response has been rather concise, but I hope that helps! If you have any questions about more specific aspects of the region's history, or if I misinterpreted and you were actually talking about library books rather than online resources, let me know and I'll be glad to follow up.