1 - Chicka Chicka Boom Boom - This is a classic for good reason- goofy, lively, rhythmic, and so much fun! It comes in board book and hard cover versions. My little and big kid both love this book (as I think does every kid on the planet ; )
2 - Learning Resources Letter Construction Set - This is another great toy both for younger kids (who will just fiddle around with it and maybe make random shapes) and for kids who are actually learning letters. I love ABC fine motor toys like this that let kids experience letters more physically than any worksheet ever could!
p.s. these are Amazon affiliate links!