There's nothing more frustrating than crochet pattern books you can't read! I once borrowed a book of crochet patterns from the library and discovered it was all in the old British system. I had to translate the patterns into modern terms before I could use them.
Books with a recent publication year will use the modern sizes in standardization for crochet hooks and yarn weight. Always rely on the gauge and make a swatch to be sure your hook will produce the size you need.
In the system for crochet hook sizes, a larger letter and number means a larger hook, which means fewer stitches per inch. The opposite is true with steel hooks. They overlap in available sizes somewhat but can be made even smaller than hooks made of other materials.
|K-10 1/2||6.5 mm||-|
In the old UK system, it was the opposite so that a smaller number meant a larger hook. If you are using vintage patterns, they might not even suggest what size hook to use. That's why it's so important to make a swatch and test the gauge before you start crocheting pattern!
These books are great for learning how to crochet and starting with some easy crochet projects then expanding to try new things.