I just made up for that Thursday level 1 backup, I can make it level 0, 2, or 3 or 4 it does not matter.

in cumulative if today´s backup level is 2 (dont get confused by these numbers, they are simply used to compare) and yesterday was 1 then I will be copying changed blocks since yesterday, if instead I had a level 2 backup yesterday then I CANNOT backup changed blocks because today´s level 2 is not bigger than yesterday´slevel 2, instead I have to go further backwards until I find a level less than my today´s level

And Sybex book is wrong I guess because with three levels you cannot do much with backups, with 5 I can set incremental backups like

level 0 -- base backup
level 1 -- monthly backup
level 2 -- weekly backup
level 3 -- daily backup
level 4 -- hourly backup