Imbue's = Fire / Lightning / Ice, these are the first set of skills you'll see in your Force Skill Tree (Ice, Lightning, Fire, Force). You use the imbue's to add magic damage to your weapon / attack
Deleveling Skills = you can delevel your skills when you hit lvl20. You'll have a quest called 'Cursed Heart'... you can gather these drops and exchange them for 'skill reset items' at the NPC (the one who sells potions)... although doing a skill reset will require you to put up some gold

to your question why would you want to 'reset' your skills... well one reason is get back the SP's you've alloted to skills you deem 'useless'... and to use those 'freed' skills to skills that you believe will help you out in SRO.
Books = when people say book 1, 2, 3... what they mean is the skill specialization you're currently at... when you look at your skill tree you'll see skills or rather a skill bracket each skill bracket will have 'books' (look at your skill tree - it'll literally look like a book LOL!) the higher the book usually means better use of that skill. Book numbering is just a guide here in the forums (since each book has names)... book 1 means the very 1st book you'll learn... then just read the books from left to right
What can you loose when you die? = well hehehe... exp... sometimes items that you are carrying (happened to me thrice already since I love deleveling my char LOL!)