He wrote this in 1864 to explain why a good Anglican and leader in the Oxford Movement chucked it and became a Catholic.
The library book sale had it for a buck, and he's fairly well known, so I tried it.
The book is his autobiography, written in less than three months as a defense against charges of being a traitor to his faith and dissembler. His attacker apparently didn't bother to research his victim, and was fairly easily trounced.
Newman's conversion to Roman Catholicism from Anglicanism would have made fewer waves if he hadn't been so prominent, but his autobiography shows that it wasn't a huge shift. In an effort to combat what he calls "Liberalism" (the ancestor of today's liberal theology--which would have made his hair stand on end), he pushed for a rethinking of Anglicanism along High Church lines that closely reflected Roman Catholic thought.
He was a High Church type of Anglican, with very high regard for Mary and for the Catholicity of the church, with a strong belief in the importance of tradition.
The biography is careful and painfully honest and interesting in places (I learned a bit about casuistry--enough to appreciate some of the problems involved in building a system that covers all the bases; though as a scientist I knew that already :-) He doesn't waste ink on discussions of obvious "heresies"(*) like Lutheranism or Calvinism, but they weren't an important part of his early life; so I suppose you have to look elsewhere to find him engaging with those churches.
The style is clause-heavy, but readable. The intellectual journey from English Catholic to Roman Catholic doesn't make for a very long trip, but his personal battles are interesting.
You can probably live long and prosper without having read the book.
(*)His word, not mine.