This is an excellent book. Ashley is well-informed about what people of differing viewpoints have to say, and this is therefore the most in-depth evangelical commentary on the book of Numbers. He doesn't accept all the conservative positions easily, but he is fairly conservative in the end.
He convincingly argues for the unity of the canonical book and undermines many source-critical "solutions" to some of the problems of interpretation. However, this doesn't mean he thinks the entire book was written by one person or during or immediately after the time of Moses (not least because the Pentateuch never suggests that it was wholly authored by Moses,and nor does any New Testament book, though Jesus does refer to them as the books of Moses the same way he refers to the Psalms as David, who clearly didn't write all of them). Ashley does think much of it goes back to Moses in some form, and he takes its own claims of its origins as genuine. He occasionally gives arguments for this about certain passages. He makes no bones about being an evangelical and seeing scripture as God's word, wholly inspired (and I assume without error in its original form, which we no longer have 100%, though he doesn't focus on the details of his views on inspiration). He doesn't take a view on problems related to large numbers in the Hebrew scriptures, but hardly anyone, evangelical or not, has a satisfying and all-encompassing view about that thorny problem.
Ashley doesn't constantly focus on theology and ties to the New Testament, but he does do a fair amount of excellent reflection on such matters in almost as much detail as his historical, linguistic, and sociological reflection.
For a more mainstream commentary, the best is Jacob Milgrom's JPS Torah commentary (which isn't just the old classic liberal viewpoint but has covered new ground, undermining lots of now-old-fashioned views still taught at the undergraduate level). Ashley had some access to Milgrom's work before revising his manuscript into the final draft, but he had little time to take into account Milgrom's whole commentary. Milgrom's thought has influenced Ashley's from his many papers and earlier books. Gordon Wenham's Tyndale volume is quite good but getting dated, and it's extremely short. Katherine Sakenfeld's International Theological Commentary and Dennis Olson's Interpretation are more recent popular level commentaries, but they're from a more critical direction. R. Dennis Cole's New American Commentary volume is more recent but isn't as detailed as Ashley's. I look forward to John Sailhamer's replacement of the Word Biblical Commentary volume by Philip Budd, but until then Ashley will be the standard for evangelicals at this level of detail. His is the most in-depth of the recent evangelical commentaries on this book, though that doesn't mean these other commentaries wouldn't complement it nicely.