I did have some little notion about Buddhism, small bits gleaned here an there, so there were several things I had simply had confirmed. Yet the majority was new to me; interesting, too, and readably written. The only two problems are (1) that the book seems to put too much emphasis on Theravāda, as compared with Mahāyāna and Vajrayāna and (2) that my memory is not what it used to be, so I have already forgotten some details. Ah well, a good excuse to read it again soon.
Postscript, 14/4/18: In the end I instead first read a much more detailed ‘introduction’ by Peter Harvey; rereading Keown last month was then a bit like reading a concise synopsis of the topic. In fact I may have only this time begun to, as it were, see the wood for the trees.