The intent as far as I can tell, is always to require proficiencies be trained by practice. However, the original idea did include the idea of having a book handy to help accelerate the learning rate from the practice. You wouldn’t read a book and just gain lockpicking, but having a lockpickers guide handy while trying to pick a lock would increase your skill gains.
Design ideas do change over time however, and Proficiency has turned into a very effective tool for encouraging actual crafting and skill practicing over just reading to lvl10 and sewing up the best gear in a weekend. I wouldn’t be surprised if proficiency books are no longer as much of an interest, as they are working very well to encourage players to focus on learning one skillset at a time in smaller projects. To quote Kevin in that issue,
One of the underlying problems if the system as it exists before proficiencies is that it doesn’t resemble the process of learning how to do things in any meaningful way.
The process is the whole point, you need to build out a repertoire of abilities and recipes instead of “find one item per skill level, craft untill you level”.
Edit: Of course I noticed that another issue was raised more recently to make exactly that happen still.
Edit 2 Electric Boogaloo: And implemented. Some books already provide a mitigation to the penalty.