I will make my first "Book cake", and need to stiffen a little my fondant, so the "book cover" would not sink down. I read that I should add Tylose powder to my fondant, but my local Michaels is out of it, and they are out of Gumpaste as well -(. Would Merinque powder work? Pls advise. Thank you

Use cornstarch!

Tylose added to fondant makes it into sugarpast or gumpaste which dries hard. Adding cornstarch might make the paste itself dry and might cause cracks.

Meringue powder is dried egg white usually  used instead of fresh egg white when making royal icing or gumpaste from scratch.

Gum tragacanth or tylose or cmc powder or wilton brand is gum tex  are all the same thing.

I really doubt mp will give you anything near the same results but  yes, cornstarch should.  Just use a little.... well how much depends on how much fondant. 

What I did many times was dip a lump of fondant into *dry* Wilton gp mix and knead until I could feel it tighten - sometimes having to dip several times until I got the right consistency.  Do the same w/your fondant/cornstarch -- coat the lump of fondant; knead it in and see how much tighter it feels before repeating.

Thank you, I will use a little of cornstarch then.

What a great question!  Glad you asked.  Informative answers all around.