Hello, I'm interested in buying those books, but before that I would like to know what you, Historians, that read those books or know the works of David Birmingham have to say. For example, are his works trustworthy or deliver an interesting insight of the subject? Is he a biased historian to a way of thinking that can deturp reality? Something like that. Thank you!
Portugal and Africa is perhaps a misleading title, in that it's not a comprehensive historical study of Portuguese colonialism in Africa but instead a substantial set of essays primarily focused on Angolan history. You would probably be better off reading his more recent A Short History of Modern Angola.
Decolonization in Africa is similarly less comprehensive than the title might imply, but it's a decent enough treatment of the topic.
Birmingham also edited a three-volume history of Central Africa with a lot of important essays in it.
I think generally his work is regarded as reliable and important. I think it's a bit behind the times in some respects today--there has been some important work on decolonization since his book was published that would be better to engage and on Angolan history there's a significant newer historiography by a range of authors writing in Portuguese and in English that might be rewarding to read. If you were going to spend time with Birmingham, I would look at his more recent history of Angola first.