Linda's Low Carb is great. Lots of simple recipes.
However, as for actual cookbooks my favorites are George Stella's. http://www.amazon.com/George-Stellas-Livin-Low-Carb/dp/0743269977/ref=sr_1_4?ie=UTF8&qid=1431602840&sr=8-4&keywords=george+stella Looks like there are some used ones for pretty cheap. Check out used book stores for his books. I often see them.
Dana Carpender has a couple "500 Low Carb Recipe" books that are decent too http://www.amazon.com/500-Low-carb-Recipes-Snacks-Dessert/dp/0739429736/ref=sr_1_4?ie=UTF8&qid=1431602922&sr=8-4&keywords=Dana+Carpender
And, finally, one of our own members has a website with her ebooks. http://www.ibreatheimhungry.com/cookbooks-new There is an option to buy them in PDF version and print them out. Maybe you could get that for your mom and print it out for her??? late mothers day gift, perhaps? :)
Online tends to be more common but I like these two (although they are low carb not neccesarily Keto so some recipes might need tweaked)
http://www.amazon.com/Eating-Stella-Style-Low-Carb-Recipes/dp/0743285212/ref=ntt_at_ep_dpt_2