There aren't, yet, many books about Sentientism (plenty of articles though!). There are, however, thousands of books about themes relevant to sentientism - epistemelogical and moral philosophy; religion, humanism, secularism, scepticism and atheism; consciousness and sentience; human, animal, even alien and robot ethics; artificial intelligence...
You'll find links below for our GoodReads group where you can find lots of great reading material, as well as a couple of books that do bear more directly on Sentientism.
Please feel free to suggest others we should add here - or even write one yourself!
In his new book, “Reasoned Politics“, suspected celebrity sentientist Magnus Vinding sets out a realistic vision for a politics grounded in compassion for all sentient beings (particularly a concern for their suffering) and a commitment to using evidence and reason in understanding our world.
The Expanding Circle sets out the biological and evolutionary bases for morality and how we have improved on those through use of evidence and reason - in part, to widen our circle of concern within and then beyond the human race. There's a short Wikipedia entry here.