Currency of Grammar Book
I have downloaded a free PDF Swahili grammar and vocabulary book. It was published in 1910. I appreciate that 109 years on the vocabulary will be dated but just how out of date may the grammar be? Hopefully someone with a good knowledge of Swahili will answer this rather than people just guessing by its age it will be definitely out of date. I have a grammar of English of the same vintage. Anyone learning English from it would have no difficulty being understood in 2019.
From my limited knowledge of Swahili, I'd guess it hasn't changed a lot - and much less than many languages. The reason being, as you probably know, that Swahili is the general language used by 41 of the 42 main tribes of Kenya (as well as being used in Tanzania and parts of other countries) to communicate with each other, although each has their own language as well. Therefore, by my (possibly faulty!) logic, any rapid changes in the language would be difficult and cause communication problems, particularly in a country where travel and other forms of communication have moved forward much more slowly, until very recently, than in America or Europe ( or many others).
I would guess that the change in language will be beginning to move faster now, with the rise of Sheng among younger people and improvement in communications generally. It will be interesting to see whether the changes are starting to affect the Maasai, who have generally tried to maintain their own culture and lifestyle, resisted learning Swahili (or English), and proudly kept their traditions - it would be a pity to lose their uniqueness.