Adult or adanced young adult novels with (usually anthropomorphized) animals as main characters, written in an adventure or fantasy style. I suppose the most famous would be Watership Down. More recent additions include the Ghatti books (cat-like), Redwall series (various woodland critters), Catswold Portal (erm, cats), Tailchaser's Song (cats), Piper at the Gates of Dawn and sequel(s?) (horses and dogs), The Wild Road (cats), and, I'm sure, lots of others.