Runciman/Riley-Smith are good over-views. The Crusades by Meyer is much better. I tend to prefer the German rather than British. (Although I am english.)

The Monks of War by D. Seward for the formation of the Crusader Orders and crusades in europe.

A Distant Mirror by B Tuchman is quite interesting.

The Formation Of Christendom by J Herrin (I think), is fantastic, but only goes as far as Charles the Great.

And don't forget the Alexiad by Anna Comnemnus (daughter of Alexius I of Byzantium)

But all this will only give you someone else's views. You can find a lot of primary text online. In particular, look for articals on Pope Gregory VII, and Pope Urban I. You might also want to look into the notion of charity and the investment of Bishops, the truce of God, and some of the works of St. Agustine.

Don't forget, the word crusade was invented in the 19th century. Not a single crusader every called themselves such. So what were they? Milites Christi?

And don't buy these books. Borrow them from the library. Slowly you should be able to meet like minded people who will share with you. You will find it very hit and miss after you have read the standard texts. If you read a book, it may take you say 15 hours, in the same time you might read 15 articals giving 15 peoples views. This is much better after you find you feet. This is a big subject.