Each profession has an elite and non-elite skill tome that allows the character who reads it to learn a skill for that profession that they have already unlocked elsewhere for the account. Skill tomes can stack with others of the same type and profession. It does not cost a skill point to learn a skill using a skill tome.
Skill tomes can only be used while in a town or outpost, not while in an explorable area.
You must first have the skill unlocked in order to learn it, you can achieve this by earn balthazar points in any pvp arena then using those points at the priest of balthazar to unlock the skill. Once the skill is unlocked you can use your tome to so your character can learn it. It is Cheaper to use tomes instead of purchasing skills once it cost 1 platinum to buy a skill(each time you purchase a skill from the skill trainer the price goes up and maxes out at 1,000 gp). Tomes range from 200-900gp depending on class, while monk is normally the most expensive. If you would like to purchase tombs you can do so at any of the major hubs, i.e.Kamadan, Kaineng Center Center, Lions Arch, Eye of the North.
Check this thread for current prices.
http://www.guildwarsguru.com/forum/s...php?t=10236961
Also, Elite tomes can be very useful if you need an elite skill right away and due not have the time to cap the skill yourself, Personally I always try to cap the skill unless I need it asap for a pvp game or a difficult mission and have players waiting.
Hope that helps
~Mega