Travelmaster Series, sheet 6, "Wales & West Midlands". If you only want to buy one map which covers all of Wales and the Welsh Borders, this is the one to get. At a scale of 1:250 000 (1 inch to 4 miles, 1 cm to 2.5 km) you can just about find your way around, but does not show much detail.
Landranger Series. Covers Wales and the Welsh Borders in 24 maps (each costing £5.99). At a scale of 1:50 000 (1.25 inches to 1 mile, 2 cm to 1 km) these maps show much more detail, and are the ones to get if you really want to explore the back lanes.

Guide Books:-

There are lots of cycletouring route guide books available. The Ordnance Survey Cycle Tours series is probably the best. Each spiral-bound book, costing £9.99, contains about 24 one-day routes (mainly on-road) based on the O.S. 1:50 000 maps. "North Wales and the Marches" (ISBN 0 600 59007 0), covers north Wales plus the northern Welsh Borders ('Marches' is another name for the Welsh Borders). "South, West and Mid-Wales" (ISBN 0 600 58846 7), covers southern Wales. "Gloucestershire and Hereford & Worcester" (ISBN 0 600 58665 0), covers the southern Welsh Borders. Be aware that information can change between the time a book is written and the time you read it. [My partner co-wrote the "Around Birmingham" edition (ISBN 0 600 58623 5) which covers the Midlands, just to the east of the Welsh Borders]

Bicycle Beano's cycling holiday in Snowdonia, North Wales, is featured in the book...

A Place to Cycle
– Amazing rides from around the world
by Rob Penn, published by Conran Octopus.

"the ultimate guide to the world's top 25 cycling holidays"
Read an extract

Where To Buy:-

Cyclist's Touring Club - The CTC's shop sells mail order cycletouring guide books, OS maps, and self-guided routes. E-mail: shop@ctc.org.uk.
Ordnance Survey - Buy their maps online.
Maps Worldwide - Mail order Ordnance Survey maps and cycling guides (based in UK).

Cycling Sideways is sponsored by...

Bicycle Beano Cycling Holidays

Sociable cycling holidays on idyllic country lanes in Wales and England with delicious vegetarian organic cuisine (lunchtime veggie and meat options available). Lively friendly groups. 2 to 7 days, May to September.

"Great cycling country and Bicycle Beano are warmly recommended for value for money fun" – Cycling Plus

"Bicycle Beano holidays are cycling idealism come true – the tours combine the best countryside in Wales, the quietest roads, gorgeous accommodation and a superb bounty of home-made vegetarian food." – The Sunday Times