Walking Guide Books
Check the reviews. Has the book been updated recently? Have the walks been walk checked by anyone other than the author. Are there web updates to the book?
E-Book or Paper
For a walking guide, paper is still best - its hard to "browse" an e-book. They are great for fiction, where you read each page in order, but poor for fact books, in which you're dipping in and out of the book the whole time!
'Black and White' Amazon Kindles are great for using in bright sunlight, and for battery life, but very poor for displaying maps. Kindle 'Fire' is much better for maps (but check the maps are usable - they may be 'black and white' kindle format), but battery life and sunlight are issues. For the maps reason, you'll find many guides aren't available in Kindle format.
Some publishers (worried about piracy) use a Adobe Ebook Viewer, which works on Android and Iphone tablets, and is very restrictive about copying, but I haven't tried this
I was send an Ebook in PDF format by Sunflower the day before a holiday. The file was very large, but they had tried (maps where high quality images, pictures were much less). I had some problems transferring it, so ended up using it on a laptop. Don't do it the day before you leave! But the fact its a PDF makes far more portable than the 'Adobe Viewer' - you can view it on a smartphone, PC, Kindle, or tablet!
Cicerone is a well respected publisher of "serious" walking (and climbing) guides to the English National Parks, Scotland, the Alps and beyond.
Many of their books are "the" guidebook for a particular area (e.g. their Tour de Mont Blanc book), other are the only English language guide to the area. However, do check at they do have a large catalogue, and some books haven't been updated for a while. They do solicit user feedback, so check for at least a second (or later) edition of the book.
Their southern English books have (access by car only) circular walks with 1:25,000 OS map excerpts. They seem to have been authored by their regular authors who happen to live in the south. The books I've looked at (Thames Valley, Sussex) are great for car travel, but not so good for public transport. No indication of walk testers or updates (pubs, facilities, and paths change a lot more frequently in the Southeast than on mountains!). But if you live in one of these areas, and have a car, these books are without equal.
"Landscape" Walking Guides to "package holiday" walking destinations, e.g. Madeira.
Aimed more at 'country' (less serious) walkers, the more popular guides are updated regularly - much more frequently than other guides books - and incorporate feedback. Great for walking but little tourist information (sightseeing, hotels, etc.). As always, check for a recent edition - walks can be replaced by housing estated in the med.
Walking Guides to the Alps and "package holiday" destinations, e.g. Madeira, Andalucia.
Aimed at more serious walkers, the books seem great at first, but I've had very poor experiences with them - the books don't seem to have been updated, or the walks tested by any one other than the author. Some Amazon feedback for them is poor as well. The Maderia one was was great though.
No tourist information.
"Walk!" guides to the UK and "package holiday" destinations
Contain GPS waypoint. They also do a series of maps which are excellent, and highly recommended. No tourist information
Lonely Planet and Rough Guides
These often contain ideas for walks, rather than the walks themselves, but have tourist information, so you'll probably find yourself buying ones of these as well as well a walking guide.