I have published a monograph and now would like to see it reviewed by a/some journal(s) in the field. Given that there are many other books they could publish a review on, they might not find a reviewer who is interested (I have already contacted them).
So I was thinking of asking a colleague to review the book (I have collaborated with him in the past, but we're not at the same institution). Is this ethical? Presumably he would be somewhat less likely to voice criticism than someone who doesn't know me personally. (Although this criterion would exclude a whole lot of people)
In the book fields I am familiar with there are two types of reviews. The first is the traditional peer review which is used to help decide if something is publishable. The second type of review, which I think you are asking about here, is a summarizing review.
Finding peer reviewers to judge the if a book should be published should be left up to the editor. If the editor asks for suggested reviewers, you should provide suggestions. In this case, having previously collaborated with the colleague, the editor may see this as a conflict of interest. If you suggest your colleague, you should be upfront about the prior collaboration
For reviews that summarize the work, if you know a publisher that takes unsolicited reviews, and you want a review there, then it is fine to ask people you know if they would be willing to write a review. Knowing, and even being friendly with, someone is not generally considered a conflict of interest. The issue is that the person you want to ask is a former collaborator. This might be seen as a conflict of interest. You can still ask the colleague and mention that you do not see the prior collaboration as a conflict of interest. You might want to suggest the collaboration check with the publisher prior to writing the review.