Runciman was a genius. A brilliant writer in English, whose grand ambitions never lead him astray from the most meticulous separation of fact from speculation, he was also an extraordinary polyglot. He read not only the Latin, Old French and Greek among the contemporary accounts of the Crusades, but the Arabic, Syriac, Persian, Hebrew, Gergian, Ethiopic, Slavonic, Norse and Mongolian as well, not to mention modern secondary works in many more languages still. If he shows any favoritism at all among the warring factions of the Crusades, then it is towards the Byzantine Greeks, although what looks like favoritism to me may only be due to my own ignorance. Even if I'm right about his favoring the Greeks, Runciman is still by far the most impartial historian of the Crusades known to me. He's certainly the only one who took the trouble and had the talent to read all the sources in the original. (Most people who've read widely in more than one language can probably appreciate how much tends to be lost in translation, not to mention how much is never translated at all.) As if his reading weren't enough, he often walked through the cities and over the battlefields which he describes in his works, in order to discover things which no one had yet written.
Runciman makes sweeping judgements and expresses strong opinions, although these are often decently hidden between the lines of his polite Cambridge prose. But all of his judgements and opinions have the support of the most solid scholarship.
I recommend the three-volume 'History of the Crusades'. The book 'The First Crusade' is an abridgement of the first volume, without footnotes or appendices or bibliography. In addition to the the three-volume history, I also have a copy of the abridgement 'The First Crusade', but it's the illustrated hardcover edition, ISBN 0521232554. I got it just for the pictures, many in color. The three volumes of the 'History of the Crusades' have a few black-and-white illustrations, and the paperback edition of 'The First Crusade' has no illustrations.