Book reviews are written in different styles according to who writes them and the house style of the newspaper or journal where they are published. They have also changed a great deal in the two centuries since Jane Austen’s books were being published and reviewed. If you want to try writing a review of the type that would have appeared when Pride and Prejudice was first published, an excellent source of models would be Jane Austen: The Critical Heritage, edited by B. C. Southam. This is divided into two volumes, the first of which deals with contemporary reviews of and later critical writings on Jane Austen’s books from 1811 to 1870.
If you are writing a contemporary review for publication in the culture or literary section of a periodical, you should bear in mind that reviews of classic novels in such publications normally require a hook, such as a new annotated edition, or a film or theater adaptation. To absorb some of the characteristic styles, read a few reviews in, for instance, The New Yorker, The New York Times, The New York Review of Books, The London Review of Books, or The Guardian. F. Scott Fitzgerald’s writing advice is always good: the more models you look at, the less likely you are to fall under the influence of any one of them.
A book review can be more personal than an academic essay but should still be tied to the text. Book reviewers are generally professional writers themselves and are chosen partly for their background knowledge. You may therefore want to refer to Jane Austen’s other novels and compare Pride and Prejudice with them or contrast Austen’s measured classical style with the fashion for gothic horror (remember that Mary Shelley’s Frankenstein was published within five years of Pride and Prejudice).