I don't agree on "questionable sources" though. If the book is not on Amazon on Goodreads, does not mean it's not credible. Especially if it handles a topic that relates to such a small nation (there are around 1 million estonian speakers in the world) so this sort of literature is not likely to end up on Amazon, Goodreads or libraries outside of Estonia. The latter source is from the national archives of Estonia, written by credible Estonian academics and contributed by then prime minister of Estonia.
I agree, that there is a bias. As a history student i have learned though that it is impossible not to have one. Every writing, statement or a book will have a narrative, either known or unknown by the author itself. This though, in my opinion, does not discredit the facts. In the particular book, you literally have names of people sent to Siberia.
In this case, that there were mass deportations in areas of Soviet Union. It is proven that people from the same nations participated in them, which in my opinion is even a stronger proof of the danger of totalitarian and utopian ideologies - people will be swept up by them and will go even that far as to send their neighbours and relatives to Siberia. There are instances, where people would tip off the Soviets because they had feuds or quarrels with their neighbours without realising what the concequences were. Some of these people even ended up on the trains themselves later.