But in general terms best approach may be to visit a big bookshop and have a browse through the walking guides on the shelf. Key is really if you find guide clear, concise, and easy to follow...and a style I might struggle with, you may find ideal.
Seconded, this is where you should find the best selection, and also in outdoors stores. But check user reviews of the titles on Amazon before buying anything, they will make you aware of any book that appears to be good but has deficiencies that aren't immediately obvious. For example, some of the AA guidebooks don't show maps for all of the walks, which to me is like a car with no steering wheel.
Also check whether the book's routes are circular or linear, you might prefer one to the other, and make sure it contains enough walks of the sort of length you like. I have about 25 books for my favourite area for day walks, but there are some I have not bought because their walks are too short for me, i.e. family walks and pub walks.
It might be worth looking in a public library too if you have one within reach. You'll often find slightly older stock that you can no longer buy new, but if you like a particular book you can then buy used a copy on Abebooks.co.uk