Hi Soeren,
Joining this thread late already but I found Laurie White's Infrared Photography Handbook to be very helpful when I first started shooting IR. Cheap, easy read, good overview of IR, and lots of useful information. Basic things from effects you can expect if you use a Red #25 filter in the sunlight versus with tungsten light, to more advanced topics too. This book is geared more toward roll film but a lot of the general principles apply to sheet film as well. Though this thread has a lot of great information in it too!