Books I've read on discipline and what I think of them (in hopes that it will help you chose
Unconditional Parenting - Great look into philosophy of gentle discipline/no rewards or punishments ideology. Also has practical tips. My one caveat with the book is the style that it is pretty harsh on those who use a different style of parenting, which I think could prevent some parents from hearing the message. I think it's a great book for parents of a 9 month old, though. I would generally recommend a different book if the parents had older children and had already been using a different style, as there are other books with the same message, but with less guilt (and less research).
Attached at the Heart - New book, only has one chapter for discipline, but I find it to be the most comprehensive of all the books I've read. It includes research about the use of punishments, non-violent communication, a list of practical tips, etc. I'm very fond of this book in total, and the discipline chapter is top notch. It is comprehensive, but brief (only one chapter), but could easily be a wonderful starting point--giving you glimpses of different ideas.
Respectful Parents, Respectful Kids - I find this to be the easiest book to swallow for even non-gentle disciplinarians. It takes the idea of Nonviolent Communication and applies it to family life. There is a large section in the back with games and tools for practicing family conflict resolution. I love this book! It is great to read, but may not be fully useful until children are a bit older. I've found the ideas and philosophy useful, but the tools/games would be more useful with a child who can sit and play a game with simple rules.
Nonviolent Communication - Not a parenting book, per se; but forms the basis for the above book (different authors, you don't need one for the other). I highly recommend this book for all people, especially parents. It teaches a way of communication from and to the heart to solve conflicts.
Raising Children Compassionately - A booklet, by the author of Nonviolent Communication. It applies Nonviolent Communication to families, but with less depth (and less pages) than Respectful Parents, Respectful Kids.
Montessori from the Start - I agree with an above poster saying there is a lot of good in this book, as well as plenty I disagree with. For the discipline aspects of this book, I find the general Montessori recommendations (promoting inner discipline, prepared environment, etc.) most useful. Some of the practical tips, however, I disagreed with. Specifically training young infants to sleep through the night, weaning before the child is ready, and closing a young child who is protesting in their room at night for sleep. I feel like these examples do not follow the child. However, there is so much in the book that is useful. So, I still recommend, but would like to warn to read with discretion.
Hold on to Your Kids - This book has ideas that are not found in other books (except for Attached at the Heart, which quotes it). I find this book more philosophical and less practical than others, but with useful and unique information. The main thesis is that children have one main attachment, either parents or peers. And it applies attachment theories to this thesis showing how (for example) if the main attachment between a child is peers and a parent is threatening that, then opposition to the parent occurs (and vice versa). It's interesting, and I can relate it to my teenage-hood. I'd be very interesting in hearing a "Montessorian's" view of this book.
How to Raise an Amazing Child the Montessori Way - A personal favorite, but the amount of "discipline" information is sparse. Although the inner discipline is within almost all of the content of the book (but I think more would be needed unless gentle discipline just comes naturally).
Giving the Love that Heals - Very useful for working through problems that arise in the parents with disciplining your children. It's truly fascinating, I'd recommend it for everyone, but would be most useful from those trying to parent different from their parents.
How to Talk So Kids Will Listen and Listen So Kids Will Talk - I think this book would be valuable for a parent looking for a quick fix, need help imagining what the child experiences (I think anyone could learn from these examples, though), or need an out-of-the-box reading style (there are comics!). For the most part, I think this book may simplify some of the ideas in other books mentioned above into a "do this" approach without very much of the "why do this". For instance, in this book there are specific wordings to hand out to children (which makes it unvalulable for someone looking for something quick), but in Nonviolent Communication it shows you how to formulate your wordings.
I hope that all made sense