Hi AdamJL.
If you go to Amazon.co.uk, they do it (just search for Tony Drake), but their current expectation is 4 to 6 week delivery so if you're intending to do this in April you need to order asap. If you have problems, you can work out a reasonable route if you have the OS maps, my preference is for 1:25,000. If you start at Barmouth and head for Maentwrog via the obvious mountain areas and Cym Bychan, you can then continue north towards Snowdon. I would say that 3 nights out will get you to the Youth Hostel at Plas Gwynant, a good stopping point before taking on Snowdon.
If you've got the time/inclination you could then continue over Snowdon to Pen-y-pass, over the Glyders to Ogwen, and finally the Carneddau to the coast.
Regarding wild camping, the situation in the UK, no, England and Wales (because I believe Scotland is different) is that all land is owned by someone, so technically you need the owner's permission to camp. There are 'official' camp sites along the route, but as you say, it's better to lay down wherever is convenient. For example the first campsite is at a place called Cym Bychan (it's where the photograph on the front of the guide was taken), but taking the terrain into consideration, this is a bit far for a day if you want to take in the scenery and have a pleasant day's walk rather than a hell for leather rush.
But unofficially, provided you are unobtrusive and ensure your camp site is clean when you leave, it is unusual for anyone to get too upset if you sleep the night in the mountains.
By the way, you will need to take a water filter, because although there is plenty of water about, it is dangerous to drink it unfiltered.
All the best on your walk, S.