بسم الله الرحمن الرحيم والصلاة والسلام على نبينا محمد وعلى آله وصحبه أجمعين
It certainly depends on what Islamic books and knowledge the author wants to spread. Unfortunately there is a lot of trash and useless writing which is called islamic but in fact it is against the correct methodology of the Qur'an and Sunnah as understood by the salaf us saleh (the pious predecessors). So I would definitely support the authors of authentic and pure islamic books, otherwise I am happy that one can download for free and check out whether the text is correct first before giving money to some deviated Muslim association.
Generally it is true that knowledge should be free and accessible to everyone. It is an interesting subject which touches upon copyright and the ruling on that. It also touches upon the author's intention which will obviously be more pure the less he or she requests. Like the question about giving wages to the Imam of the masjid or Qur'an teacher. Another interesting subject. As far as I know then it is fine to give a salary to the Imam but there is a difference of opinion about having a fixed moathin (caller to prayer). In Kuwait for example the Ministry of Islamic Affairs always gives a fixed salary to both an Imam and a moathin.
Another issue about books is those which are printed with a thick cover and gold decorations. This makes it much more expensive and this is also mentioned as a sign of the End times. Unfortunately Muslims have a very bad habit of decorating these books in very expensive gold and color coverings which may look good on the shelf. But maybe that is just as far as these beautiful books goes i.e. they never leave the shelf or are opened rarely. Instead you find even a big shaikh using his smartphone. So maybe it is better to purchase the book and study it thoroughly because our modern world wants us to do everything in a hurry and be connected to the internet constanntly. Reading a traditional book with ink and paper seems almost out of time and place.
Thanks for post.
And Allah Knows Best.