
By Christopher Tyerman
Arguably the best small book on the Crusades. He has a chapter on how they've been interpreted in the post-Romantic age, and how they were revived in Islamic countries (they didn't even know they happened until Kaiser Wilhelm I made a pilgrimage to Saladin's grave). Currently on sale on the Amazon.ca site for $5.50, but sells normally around $10.

Also by Tyerman.
This is a hefty companion to the VSI at over 1000 pages. You couldn't ask for a better book, offers pictures and maps. This is usually sold in bookshops instead of only online.

Edited book of primary documents and woodcut drawings. 2nd edition. Most translations predate 1950. Part of a series of historical texts published by the University of Toronto. If you can find the 1st edition, there is almost no difference to the 2nd ed. aside from the final chapter.
Must read for any crusading historian.

By Rodney Stark.
Very interesting take on the Crusades, and he gives detailed information on the state of technology in European and Christian lands and Islamic lands. I was shocked to read that the Europeans were the most advanced in the realm of metallurgy, farming (crop rotation), horsemanship, and sword fighting. This is against the current faith that Christian=backward boob and Islam=god-like.

By Thomas Asbridge.
General account, seems to be a best seller, I haven't read it yet, but the reviews say it's a good intro.

By Jonathan Riley-Smith. 3rd edition.
I have read part of the 1st ed. and it is a good read.

By Thomas F. Madden.
I think this is more of a school book (1st yr uni.) as it offers a "sweeping" history.