Jerry Bickel Chassis Fabrication
Yes you should buy a copy of the SFI specs if you want to build a chassis.
But, just to clarify, the SFI specs are for the roll cage only, not the entire chassis. And they are not really plans, more like general guidelines. Prospective chassis builders should get a copy of the SFI specs for sure, but they are not by any means a complete set of plans for a chassis, only recommended minimum tubing sizes, wall thicknesses and placements for various tubes that are critical for driver safety. The SFI specs don't address the rest of the chassis tubes that fall outside of those criteria.
In some cases the builder has options for tubing placement and configuration of the roll cage elements and those are covered in the SFI spec. For example, the front crossmember has to be one piece unless you opt for larger frame rails, then it can be 3 separate pieces.
Volume 1 of the new Bickel books references the SFI specs and suggests that the reader get a copy as well. If for no other reason than the specs change over time. I have a copy of the 25.1E spec for a full bodied tube chassis that I got in August, and it is informative but you couldn't build an entire chassis with only the information it contains. I think SFI is on 25.1F now, so it has changed in some way within the last few months (the letter is the revision level)
The bottom line is that the forthcoming Bickel books will walk you through an entire chassis build step by step, tube by tube. They are way more informative on how to build a chassis than just a copy of the SFI specs, but are not a substitute for the SFI specs. Having a copy of the SFI specs in hand while going through the Bickel books would be a good idea for anyone serious about building a chassis.