Hellboy is not an easy comic to put into a reading order, there is a chronological order, an original publishing order, and more recently the trade paper back order and also the library edition order, as well as BPRD Trades and Weird Tales Trades and guest appearances in other titles. My point is their is no perfect reading order, and so therefor i placed the comics into what i feel is a good reading order. It is the one i recommend to friends of mine just getting into Hellboy, and it is my current reading order of choice. it avoids spoilers and keeps the story flowing. Basically start with 01 (Seed of destruction) then go all the way through 27 then read through the other material section.