Peter Hermon's two-volume Hillwalking in Wales is easily the most comprehensive guide to Welsh mountains; walking guides being what they are these two are the only books you really need as far as mountain-walking in Wales goes. Even if you only want a guide for a specific part of Wales (e.g. Snowdonia) I would still recommend these as no other guides I have ever encountered come close to the level of detail on offer here.
Each mountain - 166 in total - has at least one dedicated route to the summit from start points at all of which car parking is possible. By describing the routes only as far as the summits, the reader can then make up his/her own walks by combining approaches, offering hundreds of possibilities where other mountain guides might offer only 20 or so walks. However, suggestions for combinations are also offered. The level of detail and dedication is astounding; even other guides that deal with only specific ranges or areas rarely approach the level of detail with which these books cover every 2,000'-ter in Wales. Summits below 2,000' are not included, meaning hill-walking areas such as the Preseli hills or Clwydian range are absent; however one has to have a cut-off point somewhere and the level of detail for the areas that are included easily makes up for that. Many of the routes described are absent from other guides but frequently these routes are excellent; one can only imagine the huge amount of time the author must have dedicated to exploring each nook and cranny of the Welsh mountains.
Hermon's prose is easy-going, pleasant to read and characterful and the glossy editions of these, with colour photos and maps of every route make these a pleasure for the mountain enthusiast to read for their own sake alone; even when you're not planning a walk!
A slight criticism is that the chapters on each range are organised alphabetically rather than geographically, meaning that areas adjacent to one another are quite frequently in different volumes (for example, the Carneddau are in volume 1 but the adjacent Glyderau are in volume 2) - so if you want a guide to all the hills in a given area you will need to buy both volumes. It would have made more sense to have the southern ranges in one volume and then the northern ones in another. That said, many people will probably want to buy both volumes anyway; Hermon has a way of inspiring visits to summits and ranges you'd never even heard of before!