I have scanned Thomas Anderson Henry's The Plant Alkaloids and am adding it to my permanently hosted book collection. This 804 page reference is a nice if somewhat dated mid-size reference to the extraction, occurrence, and chemistry of various alkaloids. PDF with OCR and DjVu without it are both offered.
http://bcis.pacificu.edu/~polverone/alkaloids.html (http://bcis.pacificu.edu/~polverone/alkaloids.html)