What factors contribute to the credibility of a book used as source?
It depends on how you are using the source - what claims you make based on the source.

For example, if you cite the results of some experiment written up in a paper as accepted scientific fact, then that paper should ideally be in a peer-reviewed publication, and be written by someone who is qualified to perform that experiment. (Or it should be something that you can - and have - verified independently yourself.) A blog post written by someone with no credentials in the field, describing an experiment he did, that you cannot replicate, is not a credible source.

On the other hand, if the claim you make is that "one mistake sometimes made by non-professionals studying X is lack of consideration of Y", then it would be perfectly OK to cite that same blog post as evidence that someone studying X failed to consider Y.

To take your example, if you say "Some professional game developers say that X increases user engagement with a game" or "Contributors to Y say that X increased user engagement" and cite the book, that would be fine. On the other hand, if you state "X increases user engagement with games" as a matter of scientific fact, you need to support that with reference to a credible experiment that studies the effect of X on user engagement using science (not just personal experience).