Our local library has multiple sets of "Bob Books"; that includes the 5 sets currently available and various older and earlier editions. (There have been so many different original and updated sets and levels that the Bob Books website FAQ has a chart that shows which books over the years replaced which books at the various reading levels.) The books are only 5 1/2 by 4 inches, so the library keeps them in a long plastic shoebox. I mention this because our kids just set the box on a table and snack on the books like little mental potato chips.
As a treat, I got Set 4 for the kindergartener and Set 1 for his younger brother. They keep them at their homework desks. (The kinderboy has real kindergarten homework; younger bro has pretend but very important homework.) Having their own mini-library of Bob's has been very successful. In a while I'll get Set 5 and Set 2.
These are very friendly books. They are illustrated with drawings that are just a bit fancier than stick figures. The drawings are jazzed up with splashes of watercolor here and there. They are not set in formal type. Rather, the words appear to have been hand printed. That presentation may seem a bit spare and childish, but the practical effect is that the books look very inviting rather than challenging or demanding. They are fun and silly and cheerful, and tell simple and understandable little stories.
I'm not well versed in reading education theory, but these books do not seem to be based on any rigorous, inflexible or odd theory or approach. The books just progress from very simple words to more complex reading. Set 1, (with 12 mini-books), introduces all of the alphabet letters, (except "Q", which makes sense I guess), and consists entirely of two or three letter words that can be sounded out.
By the time you get to Set 4, (8 mini-books), you have longer books, many four and five letter words, consonant blends, (nd, sn, st), and some vowel combinations, (ou, ee, oo). Set 5 is the big jump to long vowels, silent "e", sight words and the like. Then, it's off to read "War and Peace".
So, I'm very happy with these books as a reading treat, and it was a bonus that the kids led me to these books instead of the other way around. (Please note: I find this stuff interesting but I have no connection to the author or the publisher of these books, and bought them with my very own money.)