It took a long time, but over the last two decades or so, Anglo-American philosophers have begun to take Nietzsche's philosophical thought seriously. Brian Leiter's book is one of the major contributions to that development - it argues for an interpretation of Nietzsche that is clear, persuasive and well-supported with textual evidence. The book contains quite a few swipes at what Leiter views as the incoherence and misinterpretation endemic in the readings of Nietzsche by Heidegger, Foucault and Alexander Nehemas. In my own view, they're least justified in the latter case, but wholly so in the former two. The book is focused specifically on Nietzsche's 'On the Genealogy of Morality', and is best read in tandem with that work. You don't have to agree with every part of Leiter's book to nevertheless view it as a model of scholarship. It is clear, precise and engaging. It is the best book of secondary literature on Nietzsche to start with; others out there are more challenging and more right (in my view), but this is the best one to get first.
All that said, beware that Leiter is currently in the process of preparing a second edition of the book, so unless you really need to buy the book for a course on Nietzsche it might be worth waiting for that to come out. It seems likely to be in the next year or two. Other books of secondary literature on Nietzsche I can recommend:
- Maudemarie Clark, Nietzsche on Truth and Philosophy
- Bernard Reginster, The Affirmation of Life
- Christopher Janaway, Beyond Selflessness (another book primarily concerned with the Genealogy)
- John Richardson, Nietzsche's System (a harder book than the other three, in my opinion)
One other note: since you're viewing Brian Leiter's book, you may already own copies of Nietzsche's works, or know which translations to get. However, if you don't, I can recommend the edition of the Genealogy published by Hackett (it has a grey cover and the translation is by Clark and Swensen - Leiter recommends this translation too, and it is widely acknowledged to be the best one of the Genealogy out there at the moment), and the translations published by Cambridge University Press in their 'Cambridge Texts in the History of Philosophy' series (they all have green covers). I'd only recommend Penguin Classics translations if they're done by R.J. Hollingdale, and the CUP editions are generally superior in terms of their introductions, scholarly apparatuses etc. as well.